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Evidence for odour-mediated assortative mating in humans: The impact of hormonal contraception and artificial fragrances
Abstract
There is substantial evidence for assortative partner preferences in humans based on physical characteristics. In contrast, evidence suggests that olfactory preferences tend to be disassortative, with people preferring body odour of potential partners who are dissimilar at key genetic loci, perhaps to gain fitness advantage through offspring heterozygosity. We compared ratings of perceived body odour similarity of real couples with those of randomly paired 'fake' couples. Contrary to prediction, we find that odours of real partners are perceived more, rather than less, similar to each other than fake couples. However, this applied only to natural odour samples: there were no differences in similarity levels of real and fake couples' samples which were collected while wearing artificial fragrances. Furthermore, in light of suggestions that hormonal contraception (HC) disrupts disassortative odour preferences in women, we compared odour similarity among real couples in which the female partner was using or not using HC at the time when the relationship began. We find that odours of HC-using couples are of intermediate similarity between non-using and fake couples, suggesting that HC use during partner choice could affect odour-influenced assortment. We also examined the association between relationship satisfaction and perceived similarity of unfragranced odours of real couples. We found that these are positively correlated in male partners but negatively correlated in the female partners, indicative of a sex difference in the relative favourability of odour similarity in partner preference. Finally, by comparing odour similarity ratings with those given by perfumers using a novel olfactory lexicon we found evidence that similarity judgements were based on the Spicy/Animalic aspects of individual odour profiles. Taken together, our results challenge the conventional view that odour-mediated partner preferences in humans are typically disassortative.
... Odour-based mating preferences are often assumed to be disassortative, as genetic dissimilarity between mating partners is expected to increase offspring health and/or survival via heterozygosity advantage (Allen et al. 2019). However, evidence for MHC-based disassortative odour preferences is contradictory (Winternitz et al. 2017;Allen et al. 2019), and in some animal taxa, preferences for MHC-similar partners have also been reported (Bonneaud et al. 2006;Sin et al. 2015;Leclaire et al. 2017). ...
... Odour-based mating preferences are often assumed to be disassortative, as genetic dissimilarity between mating partners is expected to increase offspring health and/or survival via heterozygosity advantage (Allen et al. 2019). However, evidence for MHC-based disassortative odour preferences is contradictory (Winternitz et al. 2017;Allen et al. 2019), and in some animal taxa, preferences for MHC-similar partners have also been reported (Bonneaud et al. 2006;Sin et al. 2015;Leclaire et al. 2017). In humans, it has been suggested that odour preferences towards HLA-similar partners may be an artefact resulting from ethnic heterogeneity of the study subjects (pooling of different Fig. 3 The association between sperm motility 60 min after the initiation of the follicular fluid treatment and male-female total HLA allelic similarity. ...
Major histocompatibility complex (MHC) genes are known to mediate mate choice both at the individual and gamete level. However, it has remained unclear how different episodes of MHC-associated mate choice interact and contribute to the total selection on MHC genes. Here, we clarified this interaction in humans by performing a full-factorial experiment where 10 females first ranked the attractiveness and intensity of the body odours of 11 males. Then we studied whether female odour preferences in these same 110 male-female combinations predicted sperm performance in the presence of follicular fluid (sperm-stimulating female reproductive fluid). When analyzing the total MHC similarity (including classical and non-classical MHC genes) of the male-female combinations, we found that females preferred the body odours of MHC-similar males, but that sperm motility was positively affected by the MHC dissimilarity of the male-female combinations. No associations were found for classical MHC genes only. Furthermore, odour preferences were negatively associated with sperm motility at the end of the follicular fluid treatment. Together, our results indicate that individual and gamete-level mate choice processes may act in opposing directions and that the most attractive males are not necessarily the most optimal partners at the post-copulatory level. Finally, our findings suggest that gamete-mediated mate choice may have a definitive role in disfavouring genetically incompatible partners from fertilizing oocytes.
... Even though the human sense of smell is much less sensitive, and olfaction seems to play a much lesser role in our lives compared to that of animals, and particularly dogs, there is a surprisingly high number of publications dedicated to smell preferences in humans, compared to the extremely low number of similar publications dedicated to dogs [12][13][14][15][16][17][18][19][20][21]. Further, in other species, such as mice, in which olfaction plays a crucial role in many aspects of life, there are a number of publications related to olfaction, including smell preferences [22,23]. ...
... Bonferroni post-hoc analyses showed that, in most trials, the dogs spent significantly more time in the handler zone, with relatively little time exploring the odor swabs. However, in trials 1,11,16,21,23,26,31,36, and 37, the difference in the time spent between handler versus odor zones was not significant (Figure 2), indicating that the dogs spent more time in the smell zones. Interestingly, among the odors presented in these specific trials a positive control (food and castor odors), was always present. ...
Simple Summary
There are many products that are targeted to pet owners. One category of these products is dog repellents—strongly aromatized solutions designed to stop dogs from approaching and investigating particular areas; the second are cosmetics which should be pleasant for dogs. Dogs have a particularly sensitive sense of smell; therefore, strong scents may be very intense, and not always pleasant, stimuli. It is truly interesting, then, that canine cosmetic products often have very strong fragrances designed mostly to appeal to the dog owners, rather than to the dogs themselves. Indeed, the scents that dogs choose to put on their fur differ strongly from those of common cosmetics. Dogs choose mostly intense, animal-derived smells, such as feces or carcasses, so there is a need to differentiate between canine and human smell preferences. As there is limited scientific data related to canine smell preferences, the purpose of this study was to verify dogs’ reactions to selected scents, which can also be appealing to humans. Our study shows that dogs were more likely to interact with the scents of blueberry, blackberry, mint, rose, lavender, and linalol.
Abstract
The available evidence on dogs’ scent preferences is quite limited. The purpose of this study was to verify the canine response to selected odors that may also be preferred by humans. The experiment was performed using 14 adult dogs (10 female and 4 male) of different breeds, body size, and age (1–14 years). During the experiment, dogs were exposed to 33 odor samples: a neutral sample containing pure dipropylene glycol (control) and 32 samples containing dipropylene glycol and fragrance oils. The dog was brought to the experimental area by its handler, who then stopped at the entrance, unleashed the dog, and remained in the starting position. The dog freely explored the area for 30 s. All dog movements and behavior were recorded and analyzed. The methodology of observing the dogs freely exploring the experimental area allowed us to determine the smells that were the most attractive to them (food, beaver clothing). Our study shows that dogs interacted more frequently with the scents of blueberries, blackberries, mint, rose, lavender, and linalol.
Mate choice, and sex differences in romantic behaviours, represented one of the first major applications of evolutionary biology to human behaviour. This paper reviews Darwinian approaches to heterosexual mate assessment based on physical characteristics, placing the literature in its historical context (1871-1979), before turning (predominantly) to psychological research on attractiveness judgements based on physical characteristics. Attractiveness is consistently inferred across multiple modalities, with biological theories explaining why we differentiate certain individuals, on average, from others. Simultaneously, it is a judgement that varies systematically in light of our own traits, environment, and experiences. Over 30 years of research has generated robust effects alongside reasons to be humble in our lack of understanding of the precise physiological mechanisms involved in mate assessment. This review concludes with three questions to focus attention in further research, and proposes that our romantic preferences still provide a critical window into the evolution of human sexuality.
The use of oral contraceptives (OC) in the form of a hormonal pill has been widespread for decades. Despite its popularity and long-time use, there is still much ambiguity and anecdotal reports about a range of potential side effects. Here, we addressed the potential effect of OC use on chemosensory perception. Previous research has almost exclusively focused on olfaction, but we expanded this to the trigeminal system and the sense of taste. We used Bayesian statistics to compare the olfactory, trigeminal, and taste detection abilities between a group of 34 normal cycling women and a group of 26 women using OC. Our results indicated that odor, trigeminal, and taste thresholds were not affected by the use of OC. Moreover, neither odor perception, nor taste perception was affected; all with Bayes factors consistently favoring the null hypothesis. The only exception to these results was odor identification where Bayes factors indicated inconclusive
Although pervasive in many animal species, the evidence for major histocompatibility complex (MHC) disassortative mating in humans remains inconsistent across studies. Here, to revisit this issue, we analyse dense genotype data for 883 European and Middle Eastern couples. To distinguish MHC-specific effects from socio-cultural confounders, the pattern of relatedness between spouses in the MHC region is compared to the rest of the genome. Couples from Israel exhibit no significant pattern of relatedness across the MHC region, whereas across the genome, they are more similar than random pairs of individuals, which may reflect social homogamy and/or cousin marriages. On the other hand, couples from The Netherlands and more generally from Northern Europe are significantly more MHC-dissimilar than random pairs of individuals, and this pattern of dissimilarity is extreme when compared with the rest of the genome. Our findings support the hypothesis that the MHC influences mate choice in humans in a context-dependent way: MHC-driven preferences may exist in all populations but, in some populations, social constraints over mate choice may reduce the ability of individuals to rely on such biological cues when choosing their mates.
Individuals are thought to have their own distinctive body odour which reportedly plays an important role in mate choice. In the present study we investigated individual differences in body odours of women and examined whether some women generally smell more attractive than others or whether odour preferences are a matter of individual taste. We then explored whether levels of reproductive hormones explain women's body odour attractiveness, to test the idea that body odour attractiveness may act as a chemosensory marker of reproductive fitness. Fifty-seven men rated body odours of 28 healthy, naturally cycling women of reproductive age. We collected all odours at peak fertility to control for menstrual cycle effects on body odour attractiveness. Women's salivary oestradiol, progesterone, testosterone and cortisol levels were assessed at the time of odour collection to test whether hormone levels explain body odour attractiveness. We found that the men highly agreed on how attractive they found women's body odours. Interestingly , women's body odour attractiveness was predicted by their oestradiol and progesterone levels: the higher a woman's levels of oestradiol and the lower her levels of progesterone, the more attractive her body odour was rated. In showing that women's body odour attractiveness is explained by levels of female reproductive hormones, but not by levels of cortisol or testosterone, we provide evidence that body odour acts as a valid cue to potential fertility.
Body odours reportedly portray information about an individual’s genotype at the major histocompatibility complex (MHC, called human leucocyte antigen, HLA, in humans). While there is strong experimental support for MHCassociated mating behaviour in animals, the situation in humans is more complex. A lot of effort has been spent on testing HLA-associated odour preferences of women. To date, only very few studies have looked at HLA-linked olfactory preferences in men and these studies have revealed inconsistent results. Here, we investigate men’s HLA-associated preferences for women’s body odours. Importantly, and in contrast to previous studies, these odours were gathered at peak fertility (i.e. just before ovulation) when any HLA-associated odour preferences should be strongest. We scrutinized whether men’s preference for women’s body odours is influenced by (i) the number of shared HLA alleles between men and women, (ii) HLA heterozygosity, and (iii) the frequency of rare HLA alleles. We found that men could readily differentiate between odours they found attractive and odours they found less attractive, but that these preferences were not associated with HLA. Specifically, men did not prefer odours from women who are HLA dissimilar, HLA heterozygous, or who have rare HLA alleles. Together, these findings suggest that HLA has no effect on men’s odour preferences. © 2017 The Author(s) Published by the Royal Society. All rights reserved.
Genes of the major histocompatibility complex (MHC) in vertebrates are integral for effective adaptive immune response and are associated with sexual selection. Evidence from a range of vertebrates supports MHC-based preference for diverse and dissimilar mating partners, but evidence from human mate choice studies has been disparate and controversial. Methodologies and sampling peculiarities specific to human studies make it difficult to know whether wide discrepancies in results among human populations are real or artifact. To better understand what processes may affect MHC-mediated mate choice across humans and non-human primates we performed phylogenetically controlled meta-analyses using 58 effect sizes from 30 studies across 7 primate species. Primates showed a general trend favoring more MHC-diverse mates, which was statistically significant for humans. In contrast, there was no tendency for MHC-dissimilar mate choice, and for humans, we observed effect sizes indicating selection of both MHC-dissimilar and MHC-similar mates. Focusing on MHC-similar effect sizes only, we found evidence that preference for MHC-similarity was an artifact of population ethnic heterogeneity in observational studies but not among experimental studies with more control over socio-cultural biases. This suggests that human assortative mating biases may be responsible for some patterns of MHC-based mate choice. Additionally, the overall effect sizes of primate MHC-based mating preferences are relatively weak (Fisher's Z correlation coefficient for dissimilarity Zr = 0.044, diversity Zr = 0.153), calling for careful sampling design in future studies. Overall, our results indicate that preference for more MHC diverse mates is significant for humans and likely conserved across primates. This article is protected by copyright. All rights reserved.
Self-resemblance has been found to have a context-dependent effect when expressing preferences for faces. Whereas dissimilarity preference during mate choice in animals is often explained as an evolutionary adaptation to increase heterozygosity of offspring, self-resemblance can be also favored in humans, reflecting, e.g., preference for kinship cues. We performed two studies, using transformations of facial photographs to manipulate levels of resemblance with the rater, to examine the influence of self-resemblance in single vs. coupled individuals. Raters assessed facial attractiveness of other-sex and same-sex photographs according to both short-term and long-term relationship contexts. We found a preference for dissimilarity of other-sex and same-sex faces in single individuals, but no effect of self-resemblance in coupled raters. No effect of sex of participant or short-term vs. long-term attractiveness rating was observed. The results support the evolutionary interpretation that dissimilarity of other-sex faces is preferred by uncoupled individuals as an adaptive mechanism to avoid inbreeding. In contrast, lower dissimilarity preference of other-sex faces in coupled individuals may reflect suppressed attention to attractiveness cues in potential alternative partners as a relationship maintenance mechanism, and its substitution by attention to cues of kinship and psychological similarity connected with greater likelihood of prosocial behavior acquisition from such persons.
Cultural practices may either enhance or interfere with evolved preferences as predicted by culture-gene coevolution theory. Here, we investigated the impact of artificial fragrances on the assessment of biologically relevant information in human body odor. To do this, we examined cross-sensory consistency (across faces and odors) in the perception of masculinity and femininity in men and women, and how consistency is influenced by the use of artificial fragrance. Independent sets of same and opposite-sex participants rated odor samples (with and without a fragrance, N = 239 raters), and photographs (N = 130) of 20 men and 20 women. In female, but not male raters, judgments of masculinity/femininity of non-fragranced odor and faces were correlated. However, the correlation between female ratings of male facial and odor masculinity was not evident when assessing a fragranced body odor. Further analysis also indicated that differences in ratings of male odor masculinity between men with high and low levels of facial masculinity were not present in fragranced body odor samples. This effect was absent in ratings of female odors by both female and male raters, suggesting sex-specificity in the effects of fragrance on odor perception. Our findings suggest that women may be more attentive to these odor cues, and therefore also to disruption of this information through fragrance use. Our results show that cultural practices might both enhance and interfere with evolved preferences.
Previous research suggests that artificial fragrances may be chosen to complement or enhance an individual’s body odor, rather than simply masking it, and that this may create an odor blend with an emergent quality that is perceptually distinguishable from body odor or fragrance alone. From this, it can be predicted that a new emergent odor might be more easily identified than an individual’s body odor in isolation. We used a triangle test paradigm to assess whether fragrance affects people’s ability to distinguish between individual odors. Six male and six female donors provided axillary odor samples in three conditions (without fragrance, wearing their own fragrance, and wearing an assigned fragrance). In total, 296 female and 131 male participants selected the odd one from three odor samples (two from one donor, one from another; both of the same sex). We found that participants could discriminate between the odors at above chance levels in all three odor conditions. Olfactory identification ability (measured using Sniffin’ Sticks) positively predicted discrimination performance, and sex differences in performance were also observed, with female raters being correct more often than men. Success rates were also higher for odors of male donors. Additionally, while performance was above chance in all conditions, individual odor discrimination varied across the three conditions. Discrimination rate was significantly higher in the “no fragrance” condition than either of the fragranced conditions. Importantly, however, discrimination rate was also significantly higher in the “own fragrance” condition than the “assigned fragrance” condition, suggesting that naturally occurring variance in body odor is more preserved when blended with fragrances that people choose for themselves, compared with other fragrances. Our data are consistent with the idea that fragrance choices are influenced by fragrance interactions with an individual’s own body odor.
This paper examines the degree of similarity in assortative pairing for life history strategy, mate value, delinquency, and sensational interests for same-sex friends compared to opposite-sex romantic partners in persons across a variety of nations that are culturally quite different, in spite of some sharing the same language. Specifically, we sampled participants from: (1) Tucson, Arizona, United States of America; (2) Hermosillo, Sonora, Mexico; (3) San Jose, Costa Rica; and (4) Leicester, England. Due to the structure of the data and the nature of our research questions, we used a relatively new statistical tool called the Continuous Parameter Estimation Model (CPEM; Gorsuch, 2005), in order to estimate and compare assortative pairing coefficients between the four cultures. In all four cross-cultural western cultures replications, the inter-rater reliabilities, perceived assortative pairing coefficients, and actual assortative pairing coefficients of all four traits were generally substantial in magnitude, positive, and statistically significant among pairs of social partners as well as pairs of romantic partners.
Hormonal fluctuation across the menstrual cycle explains temporal variation in women's judgment of the attractiveness of members of the opposite sex. Use of hormonal contraceptives could therefore influence both initial partner choice and, if contraceptive use subsequently changes, intrapair dynamics. Associations between hormonal contraceptive use and relationship satisfaction may thus be best understood by considering whether current use is congruent with use when relationships formed, rather than by considering current use alone. In the study reported here, we tested this congruency hypothesis in a survey of 365 couples. Controlling for potential confounds (including relationship duration, age, parenthood, and income), we found that congruency in current and previous hormonal contraceptive use, but not current use alone, predicted women's sexual satisfaction with their partners. Congruency was not associated with women's nonsexual satisfaction or with the satisfaction of their male partners. Our results provide empirical support for the congruency hypothesis and suggest that women's sexual satisfaction is influenced by changes in partner preference associated with change in hormonal contraceptive use.
There are two basic theories explaining possible principles on which people are attracted to each other
and how individuals create sexual and romantic relationships. The theory of homogamy states that in their potential or
actual partners individuals prefer characteristics that are similar to themselves ("birds of a feather flock together").
One of the mechanisms that can cause homogamy in partners might be sexual imprinting. According to this theory, individuals prefer in potential partners traits that are similar to those of the opposite sex parent. In contrast, the theory of complementarity suggests that individuals are attracted to partners with traits that are opposite to their own ("opposites attract"). From an evolutionary perspective, homogamous preferences can be explained by the theory of kin selection and outbreeding depression avoidance, while pairing on the principle of complementarity is advantageous in terms of inbreeding avoidance. We will critically review these two theories – including their implications and empirical support – from the perspective of evolutionary psychology and human ethology. We will argue that principles of homogamy and complementarity are not necessarily mutually exclusive but, rather, are complementary.
This study investigates the commonsense view that 'people tend to marry people who look like themselves'. Various explanations of the observation of facial resemblance were considered by having younger and older raters judge the degree of facial similarity among pairs of photographs representing either actual couples, or randomly paired same age individuals of the opposite sex. The results suggest that the observation of facial resemblance among couples appears to reflect a real phenomenon. It was observed by both young and old, and in new as well as older marital relationships. The study provided no supportfor environmental co-existence, perceptual bias, or matching hypotheses as explanations for this facial resemblance. However, the results are consistent with an explanation based on the repeated mere exposure hypothesis, suggesting that through repeated exposure to their own face and to the faces of others genetically similar to themselves, individuals develop an attraction to faces similar to their own.
Mate choice and mate retention may both depend in part on the principle of homogamy, or positive assortative mating. In humans, the more similar couples are, the happier and more stable their relationships are. However, the practice of homogamy in mate selection must be balanced against the need to select qualities in a mate that are slightly different from one's own, and evolutionary theory has suggested that male dominance and female attractiveness are two particularly adaptive qualities that are sought in a mate. The present study investigated the relationship between marital satisfaction and homogamy in American, British, Chinese and Turkish couples. In addition, the present research assessed the evolutionary hypothesis that spousal ascendancies on dominance and attractiveness would relate to marital satisfaction. Cross-culturally, romantic love for ones spouse increased as a function of both homogamy and some evolutionarily predicted divergences on both dominance and attractiveness. However, marital satisfaction also benefited from some ascendancies that were contrary to the predictions of evolutionary theory, suggesting that self interest and cultural criteria may also guide preferences for spousal ascendancy. The present research provides for debate concerning the cross-cultural use of evolutionary hypotheses as predictors of marital satisfaction, while also suggesting unique cultural criteria for positive assortative mating.
Measures of four dimensions of personality (Psychoticism, Extraversion, Neuroticism, and Lie scores) and six aspects of social attitudes (to sex, taxation, militarism, politics, religion and a general conservatism scale) were obtained by mailed questionnaire from 29 691 US subjects including adult twins (n = 14 761) their parents (n = 2360), their spouses (n = 4391), siblings (n = 3184) and adult children (n = 4800). After correction for the average effects of age, sex and source of sample, familial correlations were computed for 80 distinct biological and social relationships. The data allow for the estimation of the additive and non-additive effects of genes, assortative mating, vertical cultural inheritance and other non-parental effects of the shared environment on differences in personality and social attitudes. The interaction of genetic and environmental effects with sex may also be analyzed. Model-fitting analyses show that personality and social attitude measures differ markedly in major features of family resemblance. Additive and dominant genetic effects contribute to differences in both personality and attitudes, but the effects of the family environment, including vertical cultural transmission from parent to child, are much more marked for social attitudes than for personality. There is substantial assortative mating for social attitudes and almost none for personality. The causes of family resemblance depend significantly on sex for almost every variable studied. These findings clarify and extend the more tentative findings derived from previous twin, family and adoption studies.
Pair formation, acquiring a mate to form a reproductive unit, is a complex process. Mating preferences are a step in this process. However, due to constraining factors such as availability of mates, rival competition, and mutual mate choice, preferred characteristics may not be realised in the actual partner. People value height in their partner and we investigated to what extent preferences for height are realised in actual couples. We used data from the Millennium Cohort Study (UK) and compared the distribution of height difference in actual couples to simulations of random mating to test how established mate preferences map on to actual mating patterns. In line with mate preferences, we found evidence for: (i) assortative mating (r = .18), (ii) the male-taller norm, and, for the first time, (iii) for the male-not-too-tall norm. Couples where the male partner was shorter, or over 25 cm taller than the female partner, occurred at lower frequency in actual couples than expected by chance, but the magnitude of these effects was modest. We also investigated another preference rule, namely that short women (and tall men) prefer large height differences with their partner, whereas tall women (and short men) prefer small height differences. These patterns were also observed in our population, although the strengths of these associations were weaker than previously reported strength of preferences. We conclude that while preferences for partner height generally translate into actual pairing, they do so only modestly.
Male mate choice might be based on both absolute and relative strategies. Cues of female attractiveness are thus likely to reflect both fitness and reproductive potential, as well as compatibility with particular male phenotypes. In humans, absolute clues of fertility and indices of favorable developmental stability are generally associated with increased women's attractiveness. However, why men exhibit variable preferences remains less studied. Male mate choice might be influenced by uncertainty of paternity, a selective factor in species where the survival of the offspring depends on postnatal paternal care. For instance, in humans, a man might prefer a woman with recessive traits, thereby increasing the probability that his paternal traits will be visible in the child and ensuring paternity. Alternatively, attractiveness is hypothesized to be driven by self-resembling features (homogamy), which would reduce outbreeding depression. These hypotheses have been simultaneously evaluated for various facial traits using both real and artificial facial stimuli. The predicted preferences were then compared to realized mate choices using facial pictures from couples with at least 1 child. No evidence was found to support the paternity uncertainty hypothesis, as recessive features were not preferred by male raters. Conversely, preferences for self-resembling mates were found for several facial traits (hair and eye color, chin dimple, and thickness of lips and eyebrows). Moreover, realized homogamy for facial traits was also found in a sample of long-term mates. The advantages of homogamy in evolutionary terms are discussed.
Matings between close relatives often reduce the fitness of offspring, probably because homozygosity leads to the expression of recessive deleterious alleles1, 2, 3, 4, 5. Studies of several animals have shown that reproductive success is lower when genetic similarity between parents is high4, 5, 6, 7, and that survival and other measures of fitness increase with individual levels of genetic diversity8, 9, 10, 11. These studies indicate that natural selection may favour the avoidance of matings with genetically similar individuals. But constraints on social mate choice, such as a lack of alternatives, can lead to pairing with genetically similar mates. In such cases, it has been suggested that females may seek extra-pair copulations with less related males4, but the evidence is weak or lacking4, 5. Here we report a strong positive relationship between the genetic similarity of social pair members and the occurrence of extra-pair paternity and maternity ('quasi-parasitism') in three species of shorebirds. We propose that extra-pair parentage may represent adaptive behavioural strategies to avoid the negative effects of pairing with a genetically similar mate.
Theoretical studies suggest that mating and pair formation is not likely to be random. Computer simulations suggested that sex among genetically complex organisms requires mate choice strategies for its evolutionary maintenance, to reduce excessive genetic variance produced by out-crossing. One strategy achieving this aim efficiently in computer simulations is assortative mating modeled as "self seeking like". Another one is selection of "good genes". Assortative mating increases the probability of finding a genetically similar mate, without fomenting inbreeding, achieving assortative mating without hindering the working of other mate selection strategies which aim to maximize the search for "good genes", optimizing the working of sex in evolutionary terms. Here we present indirect evidence that in a significant proportion of human reproductive couples, the partners show much higher facial resemblances than can be expected by random pair formation, or as the outcome of "matching for attractiveness" or the outcome of competition for the most attractive partner accessible, as had been previously assumed. The data presented is compatible with the hypothesis derived from computer simulations, that human mate selection strategies achieve various aims: "self seeking like" (including matching for attractiveness) and mating with the best available genes. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
Mate choice is one of the most important evolutionary mechanisms. Females can improve their fitness by selectively mating
with certain males. We studied possible genetic benefits in the obligate pair-living fat-tailed dwarf lemur (Cheirogaleus medius) which maintains life-long pair bonds but has an extremely high rate of extra-pair paternity. Possible mechanisms of female
mate choice were investigated by analyzing overall genetic variability (neutral microsatellite marker) as well as a marker
of adaptive significance (major histocompatibility complex, MHC-DRB exon 2). As in human medical studies, MHC-alleles were
grouped to MHC-supertypes based on similarities in their functional important antigen binding sites. The study indicated that
females preferred males both as social and as genetic fathers for their offspring having a higher number of MHC-alleles and
MHC-supertypes, a lower overlap with female’s MHC-supertypes as well as a higher genome wide heterozygosity than randomly
assigned males. Mutual relatedness had no influence on mate choice. Females engaged in extra-pair mating shared a significant
higher number of MHC-supertypes with their social partner than faithful females. As no genetic differences between extra-pair
young (EPY) and intra-pair young (IPY) were found, females might engage in extra-pair mating to ‘correct’ for genetic incompatibility.
Thus, we found evidence that mate choice is predicted in the first place by the ‘good-genes-as-heterozygosity hypothesis’
whereas the occurrence of extra-pair matings supports the ‘dissassortative mating hypothesis’. To the best of our knowledge
this study represents the first investigation of the potential roles of MHC-genes and overall genetic diversity in mate choice
and extra-pair partner selection in a natural, free-living population of non-human primates.
A number of recent studies use body odor samples to study how odor affects various human social interactions. However, the
methods used vary considerably, and only limited attention has been paid to the validity of the particular approaches adopted.
One of the crucial points in body odor sampling is its length. Here we report the results of a study that tested the effects
of using two different sampling periods, namely 12 and 24h, on subsequent odor perception. Our results show significantly
higher ratings of pleasantness and attractiveness, and lower ratings of intensity and masculinity, for the 12-h sampling condition.
Further analysis of the data distribution suggests higher skew and a potential floor effect in hedonic ratings derived from
the 24-h condition. These data imply that sampling length has a significant impact on perceived body odor quality and should
thus be considered when designing studies in this area and in interpretation of results across studies.
KeywordsArmpit–Attractiveness–Mate choice–Methodology–Olfaction–Smell
Our natural body odor goes through several stages of age-dependent changes in chemical composition as we grow older. Similar changes have been reported for several animal species and are thought to facilitate age discrimination of an individual based on body odors, alone. We sought to determine whether humans are able to discriminate between body odor of humans of different ages. Body odors were sampled from three distinct age groups: Young (20-30 years old), Middle-age (45-55), and Old-age (75-95) individuals. Perceptual ratings and age discrimination performance were assessed in 41 young participants. There were significant differences in ratings of both intensity and pleasantness, where body odors from the Old-age group were rated as less intense and less unpleasant than body odors originating from Young and Middle-age donors. Participants were able to discriminate between age categories, with body odor from Old-age donors mediating the effect also after removing variance explained by intensity differences. Similarly, participants were able to correctly assign age labels to body odors originating from Old-age donors but not to body odors originating from other age groups. This experiment suggests that, akin to other animals, humans are able to discriminate age based on body odor alone and that this effect is mediated mainly by body odors emitted by individuals of old age.
Cross-culturally, fragrances are used to modulate body odor, but the psychology of fragrance choice has been largely overlooked. The prevalent view is that fragrances mask an individual's body odor and improve its pleasantness. In two experiments, we found positive effects of perfume on body odor perception. Importantly, however, this was modulated by significant interactions with individual odor donors. Fragrances thus appear to interact with body odor, creating an individually-specific odor mixture. In a third experiment, the odor mixture of an individual's body odor and their preferred perfume was perceived as more pleasant than a blend of the same body odor with a randomly-allocated perfume, even when there was no difference in pleasantness between the perfumes. This indicates that fragrance use extends beyond simple masking effects and that people choose perfumes that interact well with their own odor. Our results provide an explanation for the highly individual nature of perfume choice.
Scent communication plays a central role in the mating behavior of many nonhuman mammals but has often been overlooked in the study of human mating. However, a growing body of evidence suggests that men may perceive women's high-fertility body scents (collected near ovulation) as more attractive than their low-fertility body scents. The present study provides a methodologically rigorous replication of this finding, while also examining several novel questions. Women collected samples of their natural body scent twice--once on a low-fertility day and once on a high-fertility day of the ovulatory cycle. Tests of luteinizing hormone confirmed that women experienced ovulation within two days of their high-fertility session. Men smelled each woman's high- and low-fertility scent samples and completed discrimination and preference tasks. At above-chance levels, men accurately discriminated between women's high- and low-fertility scent samples (61%) and chose women's high-fertility scent samples as more attractive than their low-fertility scent samples (56%). Men also rated each scent sample on sexiness, pleasantness, and intensity, and estimated the physical attractiveness of the woman who had provided the sample. Multilevel modeling revealed that, when high- and low-fertility scent samples were easier to discriminate from each other, high-fertility scent samples received even more favorable ratings compared with low-fertility scent samples. This study builds on a growing body of evidence indicating that men are attracted to cues of impending ovulation in women and raises the intriguing question of whether women's cycling hormones influence men's attraction and sexual approach behavior.
We review the literature on the familial resemblance of body mass index (BMI) and other adiposity measures and find strikingly convergent results for a variety of relationships. Results from twin studies suggest that genetic factors explain 50 to 90% of the variance in BMI. Family studies generally report estimates of parent-offspring and sibling correlations in agreement with heritabilities of 20 to 80%. Data from adoption studies are consistent with genetic factors accounting for 20 to 60% of the variation in BMI. Based on data from more than 25,000 twin pairs and 50,000 biological and adoptive family members, the weighted mean correlations are .74 for MZ twins, .32 for DZ twins, .25 for siblings, .19 for parent-offspring pairs, .06 for adoptive relatives, and .12 for spouses. Advantages and disadvantages of twin, family, and adoption studies are reviewed. Data from the Virginia 30,000, including twins and their parents, siblings, spouses, and children, were analyzed using a structural equation model (Stealth) which estimates additive and dominance genetic variance, cultural transmission, assortative mating, nonparental shared environment, and special twin and MZ twin environmental variance. Genetic factors explained 67% of the variance in males and females, of which half is due to dominance. A small proportion of the genetic variance was attributed to the consequences of assortative mating. The remainder of the variance is accounted for by unique environmental factors, of which 7% is correlated across twins. No evidence was found for a special MZ twin environment, thereby supporting the equal environment assumption. These results are consistent with other studies in suggesting that genetic factors play a significant role in the causes of individual differences in relative body weight and human adiposity.
Contextual cues of genetic relatedness to familiar individuals, such as cosocialization and maternal-perinatal association, modulate prosocial and inbreeding-avoidance behaviors toward specific potential siblings. These findings have been interpreted as evidence that contextual cues of kinship indirectly influence social behavior by affecting the perceived probability of genetic relatedness to familiar individuals. Here, we test a more general alternative model in which contextual cues of kinship can influence the kin-recognition system more directly, changing how the mechanisms that regulate social behavior respond to cues of kinship, even in unfamiliar individuals for whom contextual cues of kinship are absent. We show that having opposite-sex siblings influences inbreeding-relevant perceptions of facial resemblance but not prosocial perceptions. Women with brothers were less attracted to self-resembling, unfamiliar male faces than were women without brothers, and both groups found self-resemblance to be equally trustworthy for the same faces. Further analyses suggest that this effect is driven by younger, rather than older, brothers, consistent with the proposal that only younger siblings exhibit the strong kinship cue of maternal-perinatal association. Our findings provide evidence that experience with opposite-sex siblings can directly influence inbreeding-avoidance mechanisms and demonstrate a striking functional dissociation between the mechanisms that regulate inbreeding and the mechanisms that regulate prosocial behavior toward kin.
Previous research indicates that the scent of developmental stability (low fluctuating asymmetry, FA) is attractive to women who are fertile (at high-conception risk points in their menstrual cycles), but not to other women or men. Prior research also indicates that the scent of dissimilarity in major histocompatibility complex (MHC) genes may play a role in human mate choice. We studied the scent attractiveness to the opposite sex of t-shirts worn for 2 nights' sleep. Our results indicate that the two olfactory systems are independent. We repeated previous results from studies of the scent of symmetry. We repeated previous results from MHC research in part; men, but not women, showed a preference for t-shirts with the scent of MHC dissimilarity. Women's scent ratings of t-shirts were uncorrelated with the wearer's MHC dissimilarity and allele frequency, but positively correlated with the wearer's MHC heterozygosity. Fertile women did not exhibit any MHC trait preferences. Women's preference for the scent of men who were heterozygous for MHC alleles may be stronger in women who are at infertile cycle points. Men preferred the scent of common MHC alleles, which may function to avoid mates with rare alleles that exhibit gestational drive. Men also preferred the scent of women at fertile cycle points. The scent of facially attractive women, but not men, was preferred. Neither FA nor facial attractiveness in either sex correlated with MHC dissimilarity to others, MHC heterozygosity, or MHC allelic rarity. Copyright 2003.
Individuals are attuned to cues of quality in potential mates. Mate quality is assessed on both an absolute scale, independent
of the observer, and also on a relative scale, dependent on attributes of the observer. Much research has focused on how individuals
respond to either absolute or relative quality in mate choice, but how these dimensions are weighted during mate-choice decisions
is poorly understood and has recently attracted much theoretical interest. Here, we examine the interplay between women's
facial preferences for a measure of absolute quality (sexual dimorphism) and one of relative quality (self-similarity). Women
rated the attractiveness of male faces that had been simultaneously manipulated along the dimensions of masculinity and self-similarity
in short-term and long-term relationship contexts. Sexual dimorphism had a greater positive effect on ratings than self-similarity,
and masculinity and self-similarity had positive combinative effects on ratings of attractiveness. Women's coexpressed preferences
for masculine faces combined with their lesser preference for subtly self-similar faces may reflect selection of good genes,
promote optimal outbreeding, and give rise to directional selection, even in the presence of a general self-similarity preference.
Three very large, nationally representative samples of married couples were used to examine the relative importance of 3 types of personality effects on relationship and life satisfaction: actor effects, partner effects, and similarity effects. Using data sets from Australia (N = 5,278), the United Kingdom (N = 6,554), and Germany (N = 11,418) provided an opportunity to test whether effects replicated across samples. Actor effects accounted for approximately 6% of the variance in relationship satisfaction and between 10% and 15% of the variance in life satisfaction. Partner effects (which were largest for Agreeableness, Conscientiousness, and Emotional Stability) accounted for between 1% and 3% of the variance in relationship satisfaction and between 1% and 2% of the variance in life satisfaction. Couple similarity consistently explained less than .5% of the variance in life and relationship satisfaction after controlling for actor and partner effects.
House mice prefer mates genetically dissimilar at the major histocompatibility complex (MHC). The highly polymorphic MHC genes control immunological self/nonself recognition; therefore, this mating preference may function to provide "good genes" for an individual's offspring. However, the evidence for MHC-dependent mating preferences is controversial, and its function remains unclear. Here we provide a critical review of the studies on MHCdependent mating preferences in mice, sheep, and humans and the possible functions of this behavior. There are three adaptive hypotheses for MHC-dependent mating preferences. First, MHCdisassortative mating preferences produce MHC-heterozygous offspring that may have enhanced immunocompetence. Although this hypothesis is not supported by tests of single parasites, MHC heterozygotes may be resistant to multiple parasites. Journal Article
Fragrances have been used since at least 5000 years ago and all traditional scents are found in modern perfumes. Although perfumes are obviously involved in sexual communication, the significance of great individual differences in preference for fragrances is an evolutionary puzzle. The major histocompatibility complex (MHC) is a highly polymorphic and conserved set of genes that plays an important role in immune function in vertebrates. Both mice and humans have been shown to prefer the body odor of potential partners that have a dissimilar MHC genotype, which would result in heterozygous offspring. We tested whether individual preferences for perfume ingredients correlate with a person's MHC genotype. The human MHC is called HLA (human leukocyte antigen). A total of 137 male and female students who had been typed for their MHC (HLA-A, -B, -DR) scored 36 scents in a first test for use on self (“Would you like to smell like that yourself?”) and a subset of 18 scents 2 years later either for use on self or for a potential partner (“Would you like your partner to smell like that?”). An overall analysis showed a significant correlation between the MHC and the scorings of the scents “for self” in both tests. In a detailed analysis we found a significant interaction of the two most common HLAs with the rating of the 36 scents in the first study as well as with the 18 scents in the second study when evaluated for self. This result suggests that persons who share, for example, HLA-A2, have a similar preference for any of the perfume ingredients. The significant repeatability of these preferences in the two tests showed that the volunteers that had either HLA-A1 or HLA-A2 were significantly consistent in their preferences for the perfume ingredients offered. Hardly any significant correlation between MHC genotype and ratings of the scents “for partner” were found. This agrees with the hypothesis that perfumes are selected “for self” to amplify in some way body odors that reveal a person's immunogenetics.
Body odor sampling is an essential tool in human chemical ecology research. However, methodologies of individual studies vary widely in terms of sampling material, length of sampling, and sample processing. Although these differences might have a critical impact on results obtained, almost no studies test validity of current methods. Here, we focused on the effect of freezing samples between collection and use in experiments involving body odor perception. In 2 experiments, we tested whether axillary odors were perceived differently by raters when presented fresh or having been frozen and whether several freeze-thaw cycles affected sample quality. In the first experiment, samples were frozen for 2 weeks, 1 month, or 4 months. We found no differences in ratings of pleasantness, attractiveness, or masculinity between fresh and frozen samples. Similarly, almost no differences between repeatedly thawed and fresh samples were found. We found some variations in intensity; however, this was unrelated to length of storage. The second experiment tested differences between fresh samples and those frozen for 6 months. Again no differences in subjective ratings were observed. These results suggest that freezing has no significant effect on perceived odor hedonicity and that samples can be reliably used after storage for relatively long periods.
Previous studies in animals and humans show that genes in the major histocompatibility complex (MHC) influence individual odours and that females often prefer odour of MHC-dissimilar males, perhaps to increase offspring heterozygosity or reduce inbreeding. Women using oral hormonal contraceptives have been reported to have the opposite preference, raising the possibility that oral contraceptives alter female preference towards MHC similarity, with possible fertility costs. Here we test directly whether contraceptive pill use alters odour preferences using a longitudinal design in which women were tested before and after initiating pill use; a control group of non-users were tested with a comparable interval between test sessions. In contrast to some previous studies, there was no significant difference in ratings between odours of MHC-dissimilar and MHC-similar men among women during the follicular cycle phase. However, single women preferred odours of MHC-similar men, while women in relationships preferred odours of MHC-dissimilar men, a result consistent with studies in other species, suggesting that paired females may seek to improve offspring quality through extra-pair partnerships. Across tests, we found a significant preference shift towards MHC similarity associated with pill use, which was not evident in the control group. If odour plays a role in human mate choice, our results suggest that contraceptive pill use could disrupt disassortative mate preferences.
A straightforward argument is presented to calculate the number of different major histocompatibility complex (MHC) molecules in an individual that maximizes the probability of mounting immune responses against a large number of foreign peptides. It is assumed that increasing the number of MHC molecules per individual, n, has three different effects: (i) it increases the number of foreign peptides that can be presented; (ii) it increases the number of different T-cell receptors (TCRs) positively selected in the thymus; but (iii) it reduces the number of TCRs by negative selection. The mathematical analysis shows that n = 1/f maximizes the number of different TCRs that pass through positive and negative selection and that n = 2/f maximizes the probability to mount immune responses against a large fraction of foreign peptides. Here f is the fraction of TCRs deleted by one MHC molecule. Both results depend on approximations that are discussed in the paper. The model presented has implications for our understanding of the evolutionary forces acting on the MHC.
Humans were once considered to be microsmatic, but recent research
suggests that we can use our sense of smell to detect important, socially relevant information about conspecifics. However, much of the research conducted todate has investigated natural, fragrance free human body odours. While this is important in order to understand the evolution of olfactory communication in humans, it fails to account for the current (and historical) widespread use of artificial fragrances. In this chapter, we outline ways in which extraneous artificial fragrances may augment the underlying body odour, or ‘odour space’ of an individual, and how this might influence the perception of socially relevant information. In doing so, we describe some specific mechanisms for the interaction between body odours and artificial fragrances (blending and masking) and we discuss some recent work which has attempted to distinguish between these mechanisms.
Partner preferences are formed by several mechanisms, including an imprinting-like effect (parent-similarity) and homogamy (self-similarity). It is still unknown, however, whether these preferences remain stable throughout an individual's lifetime. We have therefore tested the consistency of mate choice in eye and hair colour both in a shortand long-term context. In other words, we tested whether people systematically choose partners with a particular eye and hair colour. We asked 1,048 respondents to indicate the eye and hair colour of themselves, their opposite-sex and same-sex parent, and all the romantic partners they had in their lives. Our results show that people consistently choose partners of a particular eye and hair colour in both short- and long-term contexts, which suggests that people do have their ‘types’. Nevertheless, the consistency was significantly higher in a long-term context than in a short-term context. Furthermore, the eye colour of one's partner was predicted by the eye colour of one's opposite-sex as well as same-sex parent, but the strongest parental effect was found when both parents had same eye colour. There were no significant results for hair colour. Our results thus suggest that preferences for eye colour are determined by the imprinting-like effect rather than by homogamy, and that they remain stable over time. These findings also indirectly support an assumption of stability of this imprinting-like effect in humans, since people consistently choose partners with their opposite-sex parent's eye-colour.
Human axillary (armpit) odors are highly diverse and have potential to reveal a wide range of individual information. This is echoed in gas chromatography findings, which show that axillary odors are comprised of many volatile compounds. Despite this, only a small number of verbal descriptors are used when investigating the perceptual qualities of body odors. We set out to develop a lexicon that would capture these perceptual qualities in more detail, working alongside perfumers and fragrance evaluators in order to benefit from their expertise in olfactory perception and semantic labeling of odors. Four experts developed a list of 15 verbal descriptors based on an exemplar set of male and female axillary samples, and then rated 62 samples (31 men and 31 women) using these. We explored the predictive value of these ratings, finding that subsets of descriptors distinguished male from female samples, appearing to be more reliable than explicit judgments of odor sex.
Practical applications
This lexicon was successful in discriminating sex of odor samples and could enable improved understanding of other perceptual qualities of human odor. For example, it could be possible to link specific perceptual qualities to specific cues (e.g., symmetry, masculinity) or to manipulate odors based on perceptual qualities in experimental settings, with direct practical implications for odor researchers. Furthermore, the existence of such a lexicon will allow body odors to be categorized for practical purposes. For example, such categorization will facilitate exploration of how fragrances, ingredients, or accords may interact with and complement different body odor types.
In human populations, assortative mating is almost universally positive, with similarities between partners for quantitative phenotypes, common disease risk, behaviour, social factors and personality. The causes and genetic consequences of assortative mating remain unresolved because partner similarity can arise from different mechanisms: phenotypic assortment based on mate choice, partner interaction and convergence in phenotype over time, or social homogamy where individuals pair according to social or environmental background. Here, we present theory and an analytical approach to test for genetic evidence of assortative mating and find a correlation in genetic value among partners for a range of phenotypes. Across three independent samples of 24,662 spousal pairs in total, we infer a correlation at trait-associated loci between partners for height (0.200, 0.004 standard error, SE) that matched the phenotypic correlation (0.201, 0.004 SE), and a correlation at trait-associated loci for BMI (0.143, 0.007 SE) that was significantly lower than the phenotypic value (0.228, 0.004 SE). We extend our analysis to the UK Biobank study (7,780 pairs), finding evidence of a correlation at trait-associated loci for waist-to-hip ratio (0.101, 0.041 SE), systolic blood pressure (0.138, 0.064 SE) and educational attainment (0.654, 0.014 SE). Our results imply that mate choice, combined with widespread pleiotropy among traits, affects the genomic architecture of traits in humans.
The influence of personality on quality of marriage was examined using causal modelling on a sample of 94 couples who had completed a quality of marriage scale and the revised short form of the Eysenck Personality Questionnaire. Irrespective of gender, quality of marriage was found to be influenced negatively by spouse's neuroticism, but the strongest influence on the quality of marriage of each spouse was the quality of marriage of the other. Altogether, the model accounted for over 60 per cent of the variance in quality of marriage, and provided an extremely close fit to the data. Finally, it was argued that valid conclusions about marital quality can only be derived from couples rather than from married individuals. 1994 The British Psychological Society
The major histocompatibility complex (MHC, called HLA in humans) is an important genetic component of the immune system. Fish, birds and mammals prefer mates with different genetic MHC code compared to their own, which they determine using olfactory cues. This preference increases the chances of high MHC variety in the offspring, leading to enhanced resilience against a variety of pathogens. Humans are also able to discriminate HLA related olfactory stimuli, however, it is debated whether this mechanism is of behavioural relevance. We show on a large sample (N = 508), with high-resolution typing of HLA class I/II, that HLA dissimilarity correlates with partnership, sexuality and enhances the desire to procreate. We conclude that HLA mediates mate behaviour in humans.
Research has shown that human partners are more similar than expected by chance on a variety of traits. Studies examining hair and eye colour show some evidence of positive assortment. Positive assortment may reflect attraction to self-similar characteristics but is also consistent with attraction to parental traits. Here, we examine self-reported partner hair and eye colour and the influence that own and parental colour characteristics have on these variables. Parental characteristics were found to correlate positively with actual partner characteristics for both men and women. Regression analysis predicting partner characteristics from maternal and paternal traits (which controls for own traits) revealed the greater importance of the opposite-sex parent over the same-sex parent in predicting both hair and eye colour of actual partners. The findings may reflect an influence of parental colour characteristics on human partner choice. Attraction to opposite-sex parental characteristics is seen in a wide variety of animals where it is usually attributed to imprinting processes in infancy. Although the mechanism is unclear and not necessarily confined to infancy, the data reported here are consistent with a somewhat analogous process to imprinting occurring in humans.
People are able to assess some personality traits of others based on videotaped behaviour, short interaction or a photograph. In our study, we investigated the relationship between body odour and the Big Five personality dimensions and dominance. Sixty odour samples were assessed by 20 raters each. The main finding of the presented study is that for a few personality traits, the correlation between self-assessed personality of odour donors and judgments based on their body odour was above chance level. The correlations were strongest for extraversion (.36), neuroticism (.34) and dominance (.29). Further analyses showed that self–other agreement in assessments of neuroticism slightly differed between sexes and that the ratings of dominance were particularly accurate for assessments of the opposite sex. Copyright © 2011 John Wiley & Sons, Ltd.
The variety of interpersonal relationships in contemporary society necessitates the development of brief, reliable measures of satisfaction that are applicable to many types of close relationships. This article describes the development of such a measure. In Study I, the 7-item Relationship Assessment Scale (RAS) was administered to 125 subjects who reported themselves to be "in love." Analyses revealed a unifactorial scale structure, substantial factor loadings, and moderate intercorrelations among the items. The scale correlated significantly with measures of love, sexual attitudes, self-disclosure, commitment, and investment in a relationship. In Study II, the scale was administered to 57 couples in ongoing relationships. Analyses supported a single factor, alpha reliability of .86, and correlations with relevant relationship measures. The scale correlated .80 with a longer criterion measure, the Dyadic Adjustment Scale (Spanier, 1976), and both scales were effective (with a subsample) in discriminating couples who stayed together from couples who broke up. The RAS is a brief, psychometrically sound, generic measure of relationship satisfaction.
Individuals in some species prefer mates carrying dissimilar genes at the major histocompatibility complex (MHC), which may function to increase the MHC or overall heterozygosity of progeny. Here I review the evidence for MHC-dependent mating preferences from recent studies, including studies on the underlying olfactory mechanisms and evolutionary functions. Many studies indicate that MHC genes influence odour, and some work is beginning to examine the potential role of MHC-linked olfactory receptor genes in mating preferences. MHC-dependent mating preference increases the MHC-heterozygosity of progeny, which is suspected to confer resistance to infectious diseases. In humans, heterozygosity at MHC loci is associated with increased resistance to hepatitis and HIV infections, but experimental evidence for the heterozygote advantage hypothesis has been lacking. Here I re-analyse data from previously published experimental infection studies with mice. I show that although overdominance is rare, resistance is often dominant, suggesting that heterozygotes are often protected. A second (nonmutually exclusive) possibility is that MHC-disassortative mating preferences promotes inbreeding avoidance. This hypothesis is supported by recent evidence that MHC genes play a role in kin recognition, and that mating with close kin has rather deleterious fitness consequences. In conclusion, I discuss other ways that MHC genes might influence sexual selection. The research on MHC-mediated mating preferences is integrating the study of animal behaviour with other seemingly disparate fields, including sensory biology and immunogenetics.
Females of a number of primate species display their fertile period by behavioural and/or morphological changes. Traditionally, the fertile period in human females has been considered to be concealed. However, this presumption has rarely been tested. One of the possible mechanisms for assessing menstrual cycle phase is through the sense of smell. In this study possible changes in odour across the menstrual cycle were investigated. Samples of body odour were acquired from 12 women (aged 19–27 yr), none of whom were using hormonal contraceptives. Samples were collected using cotton pads worn in the armpit for 24 h, from the menstrual, follicular and luteal cycle phases. Our experimental sample of 42 males (age 19–34 yr) repeatedly rated these odour samples for their intensity, pleasantness, attractiveness and femininity. Raw subjective smell ratings from each man were transformed to z-scores. Subsequently, these z-scores were tested by the general linear mixed-model analysis (PROC MIXED, SAS) with the female's ID nested within the subject's ID as a random factor to account for the repeated measures of the subjects. Significant changes across the cycle were found for ratings of pleasantness [F(2,689) = 702; p = 0.001], attractiveness [F(2,546) = 6.35; p = 0.002] and intensity [F(2,530) = 3.57; p = 0.028]. Odour from women in the follicular (i.e. fertile) phase was rated as the least intense and the most attractive. Subsequent post hoc analysis revealed significant differences in intensity, pleasantness and attractiveness between the menstrual phase and the follicular phase, and in pleasantness and attractiveness between the menstrual and luteal phases. Significant difference between the follicular and the luteal phase was found only for attractiveness. Our results suggest that men can potentially use smell as a mechanism for monitoring menstrual cycle phase in current or prospective sexual partners. Therefore, the fertile period in humans should be considered non-advertized, rather than concealed.
Similar criteria may operate in mate choice and in mate retention. For example, United States couples tend to be similar, and the more similar they are, the happier and more stable their relationships are. Another widespread criterion is male dominance, which females in several primate species seem to find desirable in a mate. Defined in various ways, dominance seems to characterize men that women find desirable. Also, cross-cultural evidence suggests that attractiveness, particularly in women, enhances mate value. A survey of over 1000 British couples was undertaken to test the homogamy (similarity), male dominance, and female attractiveness hypotheses in that society. In 19 of 42 tests, homogamous couples tended to be significantly (p < 0.01) more satisfied. Couples, especially wives, were more satisfied if the husband dominated decision making, but excessive husband dominance reduced satisfaction. Husbands were more satisfied if the wife was moderately more attractive than they were. Unlike some previous U.S. studies, this one revealed no relationship between marital satisfaction and the husband's earning more than the wife, being better educated, or having wealthier parents. In addition to the homogamy hypothesis, the notion that dominant men gain attractive wives received qualified support. Economic factors may be less fundamental to marital satisfaction than these other variables.
Previous theoretical and empirical studies have shown that individuals may act to the benefit of others of similar genotype. We argue that the ability to discriminate among individuals of varying degrees of relatedness is prevalent in many species, and that the tendency to favor relatives can be considered a special case of a tendency to favor those of similar genotype. The phenomenon of assortative mating can be explained in this way, but new evidence capable of disproving this conjecture is not easily obtained. We have reanalyzed three previously reported studies of heritability and assortive mating in humans, and show that there is a greater degree of assortative mating on more highly heritable traits, in accordance with the prediction.
Dating and married couples show comparable levels of assortment for physical traits. Married couples, however, are more assorted on psychological traits. It is argued that both dating and married couples initially assort on physical similarity, but that couples who are similar on psychological traits are more likely to marry. Physical traits are apparently critical in initial partner selection; psychological traits are more important for long-term relationships. There is little evidence that couples become more similar in psychological traits over time, implying that existing similarities are due to initial assortment.
Whereas the hypothesis of genetically mediated homogamy has been supported by several studies, certain theoretical and methodological criticisms have been raised against genetic similarity theory. As an alternative approach to assortative mating, we suppose that imprinting-like mechanisms, rather than “direct” genetic detection, are responsible for choosing similar spouses. In a study aimed at comparing more than 300 facial photographs of family members and controls, the judges correctly matched wives to their mother-in-law at a significantly higher rate than expected by chance. Furthermore, a higher degree of similarity was ascribed between the husbands’ mother and the husbands’ wife than between the husbands and their wives. A regression analysis has revealed that men who had been more frequently rejected by their mother during childhood were less likely to choose mates who resemble their mothers in physical appearance. These results suggest that under the influence of childhood experiences, sons internalize their mother’s phenotype as a template for acquiring similar mates.
Synopsis
It has become difficult to find a matching perfume. An overwhelming number of 300 new perfumes launch each year, and marketing campaigns target pre‐defined groups based on gender, age or income rather than on individual preferences. Recent evidence for a genetic basis of perfume preferences, however, could be the starting point for a novel population genetic approach to better match perfumes with people’s preferences. With a total of 116 participants genotyped for alleles of three loci of the major histocompatibility complex (MHC), the aim of this study was to test whether common MHC alleles could be used as genetic markers to segment a given population into preference types. Significant deviations from random expectations for a set of 10 common perfume ingredients indicate how such segmentation could be achieved. In addition, preference patterns of participants confronted with images that contained a sexual communication context significantly differed in their ratings for some of the scents compared with participants confronted with images of perfume bottles. This strongly supports the assumption that genetically correlated perfume preferences evolved in the context of sexual communication. The results are discussed in the light of perfume customization.
Extremely high variability in genes of the major histocompatibility complex (MHC) in vertebrates is assumed to be a consequence of frequency-dependent parasite-driven selection and mate preferences based on promotion of offspring heterozygosity at MHC, or potentially, genome-wide inbreeding avoidance. Where effects have been found, mate choice studies on rodents and other species usually find preference for MHC-dissimilarity in potential partners. Here we critically review studies on MHC-associated mate choice in humans. These are based on three broadly different aspects: (1) odor preferences, (2) facial preferences and (3) actual mate choice surveys. As in animal studies, most odor-based studies demonstrate disassortative preferences, although there is variation in the strength and nature of the effects. In contrast, facial attractiveness research indicates a preference for MHC-similar individuals. Results concerning MHC in actual couples show a bias towards similarity in one study, dissimilarity in two studies and random distribution in several other studies. These vary greatly in sample size and heterogeneity of the sample population, both of which may significantly bias the results. This pattern of mixed results across studies may reflect context-dependent and/or life history sensitive preference expression, in addition to higher level effects arising out of population differences in genetic heterogeneity or cultural and ethnic restrictions on random mating patterns. Factors of special relevance in terms of individual preferences are reproductive status and long- vs. short-term mating context. We discuss the idea that olfactory and visual channels may work in a complementary way (i.e. odor preference for MHC-dissimilarity and visual preference for MHC-similarity) to achieve an optimal level of genetic variability, methodological issues and interesting avenues for further research.