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1P
2Parental Investment Theory (Middle-
3Level Theory in Evolutionary
4Psychology)
5Xiao-Tian Wang
6PsychologyAu1 Department, University of South
7Dakota, Vermillion, SD, USA
8Synonyms
9Parental care, parenting;Resource provision
10 Definition
11 ParentalAu2 investment is referred to as any expendi-
12 ture (time, energy, resources, etc.) that a parent
13 incurs to benefit an offspring. Parental investment
14 theories in evolutionary biology and evolutionary
15 psychology explore mechanisms underlying
16 parent-offspring relationships and investment.
17 These theories focus on adaptiveAu3 functions, fitness
18 dynamics of parent-offspring relationship, and
19 how environmental conditions, sex differences in
20 provision of reproductive resources regulate
21 parental investment in sons and daughters.
22 Introduction
23 Throughout hominid evolution, our ancestors
24 have always managed to do two things: survive
25to reproductive age and reproduce. Human sur-
26vival and reproduction are characterized by inter-
27nal fertilization, long-term gestation, lactation,
28and prolonged infancy that requires intense paren-
29tal care and investment. Parental investment,in
30evolutionary biology and evolutionary psychol-
31ogy, is referred to as any expenditure (time,
32energy, resources, etc.) that a parent incurs to
33benefit an offspring (Trivers 1972).
34From an evolutionary perspective, offspring
35are genetic vehicles for their parents. Without
36children, an individual’s genes would perish. It is
37thus reasonable to expect that natural selection
38would favor powerful mechanisms in parents
39that ensure the survival and reproductive success
40of their children. Parental investment is thus piv-
41otal for both offspring survival and perpetual
42reproductive success of the parents. Parental
43investment is viewed as a cost in parental fitness,
44via either direct physical or physiological expen-
45diture or indirect opportunity cost in mating and
46reproduction.
47Fisher’s Principle
48A pioneering analysis of parental investment can
49be traced back to Ronald Fisher. In his 1930 book
50The Genetical Theory of Natural Selection, Fisher
51developed an evolutionary model to explain why
52the sex ratio of most species that produce off-
53spring through sexual reproduction is approxi-
54mately 1:1 between males and females.
#Springer International Publishing Switzerland 2016
T.K. Shackelford, V.A. Weekes-Shackelford (eds.), Encyclopedia of Evolutionary Psychological Science,
DOI 10.1007/978-3-319-16999-6_3585-1
55 Fisher’s model suggests that parental invest-
56 ment should also covary with this dynamic of
57 evolutionary equilibrium. Fisher built his argu-
58 ment in terms of parental expenditure and
59 predicted that parental expenditure may deviate
60 from the 1:1 ratio but would be “corrected”back
61 to an equal distribution by natural selection. Sup-
62 pose females outnumber males in a population
63 due to a war situation. A newborn male then
64 receives more parental investment and would
65 have better mating prospects than a newborn
66 female and therefore can expect to have more
67 offspring. Therefore, parents who are genetically
68 more disposed to producing males would have
69 more than average numbers of offspring. As a
70 result, male to female sex ratio becomes higher
71 than 1:1. The demand and supply relationship thus
72 is changed and then reversed: the shortage of
73 males is replaced by a shortage of females. The
74 male to female sex ratio is then downregulated
75 toward the 1:1 equilibrium. Similarly, in situations
76 that female births are less common than male
77 births, the sex ratio would be regulated down
78 toward the equilibrium (Hamilton 1967).
79 Regulated by natural selection, the total paren-
80 tal investment incurred will be equal across sex
81 for children. This implication from Fisher’s prin-
82 ciple stands in contrast to persistent differential
83 parental investment in sons and daughters. One
84 way to understand the coexistence of the 1:1 sex
85 ratio and differential parental investment is to
86 view the former as population equilibrium and
87 the latter as individual strategies that are depen-
88 dent upon situational factors and the life-stage of
89 the parents.
90 Sex ratio of an entire population is not as
91 sensitive as operational sex ratio to natural selec-
92 tion and sexual selection. Operational sex ratio is
93 a measure that is more sensitive to sexual selec-
94 tion. It is the ratio of sexually competing males
95 that are ready to mate to sexually competing
96 females that are ready to mate (Clutton-Brock
97 2007). This is a measure of how intense sexual
98 competition is in a population and the potential
99 rate of reproduction. This ratio reflects the sever-
100 ity of intrasexual mating competition and is con-
101 trolled by relative parental investment (Gwynne
102 1990). For example, if females spend more time
103caring for young than mating, but males do the
104opposite, then more males would be ready to
105mate, thus creating a male-biased operational sex
106ratio.
107Hamilton’s Inclusive Fitness (Kin
108Selection) Theory
109Parental investment theory can be viewed as a
110branch of kin selection theory. Building upon
111Fisher’s(1930) work, W. D. Hamilton (1964)
112proposed his inclusive fitness theory, which
113argues that altruistic acts toward neighbors who
114have copies of the helper genes would be favored
115by natural selection. Inclusive fitness theory holds
116the keys to understanding differential investment
117behaviors, including parental investment. The
118mathematical treatment of the theory (C rB) is
119now often referred to as Hamilton’s rule, which
120shows that an “altruistic design”can spread
121through the population if it causes an individual
122to help a kin member whenever the cost (C) to the
123helper’s own reproduction is offset by the benefit
124(B) to the recipient’s reproduction, weighted by
125the genetic relatedness between the two (r).
126Hamilton’s(1964) inclusive selection theory
127has been a powerful source for generating hypoth-
128eses about altruistic behaviors in social interac-
129tions, for which standard economic models fail to
130account. Hamilton (1964, p. 19) predicted that:
131“The social behavior of a species evolves in such a
132way that in each distinct behavior-evoking situa-
133tion the individual will seem to value his neigh-
134bor’sfitness against his own according to the
135coefficients of relationship appropriate to that sit-
136uation.”For kin selection to work, a prerequisite is
137that “a suitable social object is available”in an
138interactive social context (Hamilton 1987, p. 420).
139Parent-offspring interaction thus might be the
140most typical case for kin selection.
141Hamilton’s rule also suggests extended forms
142of parenting from kin who are not biological par-
143ents. For example, Gorrell et al. (2010) showed
144that surrogate mothers of red squirrels adopt
145related orphaned squirrel pups but not unrelated
146orphans. The authors of this study calculated the
147cost of adoption by measuring a decrease in the
2 Parental Investment Theory (Middle-Level Theory in Evolutionary Psychology)
148 survival probability of the entire litter after
149 increasing the litter by one pup and the benefitof
150 adoption as the increased chance of survival of the
151 orphan. More precisely, females adopted orphans
152 when rB >C but never adopted when rB <C.
153 Another form of extended parenting can be
154 seen in eusociality (true sociality) which is char-
155 acterized by overlapping generations between
156 parents and their offspring, cooperative brood
157 care, and the specialized castes of
158 nonreproductive individuals (Freeman and
159 Herron 2007). Social insects provide good exam-
160 ples of extended parenting. The workers of some
161 species are sterile, a trait that would not occur if
162 individual selection was the only process at work.
163 The relatedness coefficient ris abnormally high
164 between the worker sisters due to haplodiploidy,
165 whereby males are haploid and females are dip-
166 loid. This ensures that sisters are more related to
167 each other than they ever would be to their own
168 offspring. Hamilton’s rule readily accounts for
169 such extended parenting behavior, though there
170 are alternative explanations (see Nowak
171 et al. 2010).
172 Humans also exhibit extended forms of parent-
173 ing. A study of childcare practices among Cana-
174 dian women found that respondents with children
175 provide childcare reciprocally with nonkin. The
176 cost of caring for nonkin was balanced by the
177 benefit a woman received –having her own off-
178 spring cared for in return. However, for individ-
179 uals without their own offspring, the inclusive
180 fitness benefits of providing care to closely related
181 children might outweigh the time and energy costs
182 of childcare. Indeed, the respondents without chil-
183 dren were significantly more likely to offer
184 childcare to kin only (Davis and Daly 1997).
185 Trivers’Model of Parental Investment
186 Inspired by seminal work of Hamilton (1964),
187 Robert Trivers, then a graduate student at Harvard
188 University, developed a theory of parental invest-
189 ment as a result of sexual selection. In iteroparous
190 species, where individuals may go through sev-
191 eral reproductive bouts during their lifetime, a
192 tradeoff may exist between investment in current
193offspring and future reproduction. The impor-
194tance of parental investment can be seen espe-
195cially in species in which the offspring are
196altricial (i.e., unable to fend for themselves from
197earliest ages). In many bird species and in modern
198humans, this leads to males spending more time
199caring for their offspring than do the male parents
200of precocial species, since reproductive success
201would otherwise suffer.
202Based on this cost–benefit analysis, Trivers
203(1972)defined the term parental investment to
204mean any investment by the parent in an individ-
205ual offspring that increases the offspring’s chance
206of surviving (and hence reproductive success) at
207the cost of the parent’s ability to invest in other
208offspring. Overall, parents maximize the differ-
209ence between the benefits and the costs, and
210parental care will be likely to evolve when the
211benefits exceed the costs.
212The evolution of sex differences in parental
213investment is widely attributed to anisogamy
214(i.e., sexual reproduction involving two types of
215gametes that differ in size). The initial asymmetry
216in premating parental investment (eggs vs. sperm)
217is assumed to promote even greater divergence in
218postmating parental investment (parental care). In
219such a situation, a male’s reproductive success is
220limited by his ability to fertilize eggs with his
221sperm in the same-sex mating competition.
222A female’s reproductive success is limited by her
223ability to produce eggs, which requires a large
224investment of metabolic energy (Trivers 1972,
225p. 138).
226In humans, parental investment starts from the
227point when the sperm fertilizes the egg. Following
228fertilization, the minimal obligatory parental
229investment for the female is 9 months of preg-
230nancy, followed by delivery. In contrast, the min-
231imal obligatory parental investment for the male is
232almost zero. This difference of minimal obligatory
233investment between males and females suggests
234that the amount of investment and effort put into
235mating and parenting would also differ.
236Trivers (1972) introduced two arguments to
237link premating and postmating investment. The
238first argument is that females are more committed
239than males to providing care because they stand to
240lose a greater initial investment. The second
Parental Investment Theory (Middle-Level Theory in Evolutionary Psychology) 3
241 argument takes the reasonable premise that
242 anisogamy produces a male-biased operational
243 sex ratio leading to males competing for mates.
244 Male care is then predicted to be less likely to
245 evolve as it consumes resources that could other-
246 wise be used to increase competitiveness.
247 These two arguments however are challenged
248 by other scholars (see Kokko and Jennions 2008).
249 The first argument has been criticized for commit-
250 ting the “Concorde fallacy”as optimal decisions
251 should depend on future payoffs, rather than past
252 costs. However, the argument can be made in
253 terms of residual reproductive value when past
254 investment affects future payoffs. The second
255 argument was challenged by the Fisher principle.
256 Given each offspring has precisely two genetic
257 parents, a biased operational sex ratio would gen-
258 erate frequency-dependent selection that favors
259 increased parental investment by the sex facing
260 more intense competition, in this case males.
261 Although facing some theoretical challenges,
262 predictions from Trivers’(1972) model received
263 ample empirical support. Using measures from
264 time spent in vicinity, touching, and teaching chil-
265 dren shows that women indeed care for their chil-
266 dren more intensively than men do (Geary 2000).
267 Besides the mating opportunity costs hypothe-
268 sis suggested above, another mechanism underly-
269 ing the phenomenon that mothers invest more
270 than fathers in offspring is the paternity uncer-
271 tainty hypothesis. Mothers, consciously or not,
272 are “sure”of their genetic contribution to their
273 offspring (maternity certainty). When a female
274 gives birth or lays a fertilized egg, there is no
275 doubt that her offspring will contain 50 % of her
276 genes. In contrast, from a male’s perspective,
277 there can always be some probability that the
278 offspring was fathered by another male
279 (paternity uncertainty).
280 Parent-Offspring Conflict
281 Given the importance of offspring, one of the
282 astonishing facts about parental care is that many
283 species do not engage in it at all (Alcock 2001).
284 Part of the reason for the lack of universality of
285 parental care is that it is so costly. By investing in
286offspring, parents lose out on resources that could
287be devoted to themselves or toward finding addi-
288tional mates. Parents who protect their young risk
289their own survival. Thus, when fitness cost is
290higher than fitness benefit, a stopping mechanism
291becomes necessary.
292From an evolutionary perspective, parents and
293children are predicted to have conflicts. Following
294his own work on parental investment and sexual
295selection, Trivers (1974) developed a model of
296parent-offspring conflict. In sexually reproducing
297species such as humans, parents and offspring are
298genetically related by 50 %. This genetic related-
299ness between parent and child can exert selection
300pressure for parental care. But it also means that
301parents and children differ genetically by 50 %.
302Fitness benefit for one is not perfectly correlated
303with fitness benefit for the other. Specifically, par-
304ents and children will diverge in the ideal alloca-
305tion of the parents’resources, the typical result
306being that children want more for themselves than
307parents want to give (Trivers 1974).
308For a parent of two offspring with two units of
309food available, the optimal allocation would be to
310give one unit of food to each offspring. Consider-
311ing that there is a diminishing return associated
312with each additional consumption, the value of the
313first unit of food consumed is higher than the
314value of the second unit of food. However, for
315each offspring, the ideal allocation is to get two
316units of food. The theory of parent-offspring con-
317flict predicts that each child will generally desire a
318larger portion of the parents’resources than the
319parents want to give. In addition, parent–child
320conflict over the parents’resources is predicted
321to occur not merely at particular times such as
322adolescence, but at each stage of life.
323Trivers’theory (1974) identified an important
324area of genetic conflict of interest between parents
325and children. Over evolutionary time, there will
326be an “arms race”between the genes expressed in
327parents and the genes expressed in children.
328Selection is therefore predicted to forge adapta-
329tions in children to manipulate parents toward the
330children’s optimum resource allocation and
331counter-adaptations in parents to manage resource
332allocation toward their own optimum. This theory
333yields some surprising predictions. For example,
4 Parental Investment Theory (Middle-Level Theory in Evolutionary Psychology)
334 mother-offspring conflict will sometimes occur in
335 utero, such as over the blood supply to the fetus,
336 the size of the fetus, and whether the fetus is
337 spontaneously aborted. Empirical evidence sup-
338 ports these predictions (Haig 1993).
339 r/K Selection Hypothesis and Life-
340 History Theory
341 The other overarching theory that is closely rele-
342 vant to parental investment is the r/K selection
343 theory by MacArthur and Wilson (1967).
344 According to the theory, when resources are suf-
345 ficient to carry or support organisms living in an
346 environment, organisms would be able to freely
347 exploit rich resources for fast development and
348 early reproduction that is characterized by a
349 large number of offspring with little parental
350 investment. This is known as r selection, with r
351 representing the maximal reproductive rate of a
352 species. In contrast, when resources become
353 scarce, organisms adopt slow strategies, also
354 known as K selection, with K representing the
355 carrying capacity of the environment.
356 K selection favors slow development and intense
357 parental care and investment to reproduce few,
358 high-quality offspring who can compete for and
359 monopolize the depleting resources of the
360 environment.
361 Species with K-selected traits, such as large
362 body size, long life expectancy, and the produc-
363 tion of fewer offspring, which often require exten-
364 sive parental care until they mature, include
365 elephants, humans, and whales. Although some
366 organisms are identified as primarily r- or
367 K-strategists, the majority of organisms do not
368 follow this pattern (Hrdy 2000). The r/K dichot-
369 omy can be expressed as a continuous spectrum
370 using the economic concept of discounted future
371 returns, with r-selection corresponding to large
372 discount rates and K-selection corresponding to
373 small discount rates (Reluga et al. 2009).
374 The r/K selection theory has been challenged
375 by its lack of empirical support and led to a newer
376 paradigm of life-history theory that focuses on
377 age-specific strategies and incorporates many of
378the themes important to the r–K paradigm
379(Reznick et al. 2002).
380Life-history theory provides an evolutionary
381and psychobiological framework for studying
382parental investment. The theory primarily con-
383cerns tradeoffs among various survival and repro-
384ductive needs given finite energy and resources
385and infinite inter- and intra-species competition
386(Stearns 1992). The most essential tradeoffs are
387between somatic effort (growth, learning, and
388socialization) and reproductive effort. Reproduc-
389tive effort itself involves the additional tradeoff
390between mating effort that results in offspring
391quantity and parenting effort related to offspring
392quality.
393Calibrated in terms of fitness or reproductive
394success, all tradeoffs can be summarized as the
395tradeoff between current or early reproduction and
396future or delayed reproduction. Varying along the
397fast-slow continuum, differences in life-history
398strategies result in the vast individual differences
399in observed behavior (Del Giudice and Belsky
4002011). Variations in life-history strategies are the
401result of environmental conditions. Two overarch-
402ing conditions relevant to life-history strategies
403are the extent of environmental harshness and
404unpredictability regarding mortality and mobility.
405High levels of extrinsic mortality and mobility
406lead to the adoption of fast life-history strategies
407with low parental investment because fitness is
408maximized by accelerating development and ini-
409tiating early reproduction before extrinsic mortal-
410ity or mobility hits. Conversely, low levels of
411environmental harshness and unpredictability do
412not necessarily predict slow life-history strategies
413but depend on other factors in the environment,
414such as resource levels and the extent of intraspe-
415cific competition over resources (Ellis et al. 2009).
416The most effective forms of signals in a child’s
417environment are parental behavior and related
418information, such as the absence or change of
419parental figures (Ellis 2004). The mere experience
420of changes in parental and other adult figures has
421been found to be a robust predictor of fast life-
422history strategies (Tither and Ellis 2008).
423Studies of life-history strategies suggest that
424quality of parental investment affects the adoption
425of fast or slow strategies by the offspring, which in
Parental Investment Theory (Middle-Level Theory in Evolutionary Psychology) 5
426 turn determines the parental investment strategies
427 of the offspring when s/he becomes a parent in the
428 future.
429 Differential Parental Investment in Sons
430 and Daughters
431 Trivers-Willard Hypothesis
432 Assuming an equal sex ratio in the population,
433 sons and daughters, on average, have equal repro-
434 ductive success. But the condition of the son or
435 daughter might result in differential parental
436 investment in sons versus daughters. This is the
437 core insight of the Trivers-Willard hypothesis
438 (1973). According to this hypothesis, rich parents
439 are more likely to have successful offspring and
440 should favor sons since there is a greater opportu-
441 nity for their sons to be rich and to have more
442 offspring. Conversely, poor parents should prefer
443 daughters to sons because daughters are not as
444 likely to be reproductively unsuccessful as sons.
445 Stated differently, if being in “good”condition
446 affects male reproductive success more than
447 female reproductive success, as we would expect
448 in a polygynous mating system, then parents
449 should bias investment toward sons if the parents
450 are in good condition and toward daughters if the
451 parents are in poor condition.
452 Tests of the Trivers-Willard hypothesis in
453 humans have proved inconclusive (Keller
454 et al. 2001). A few studies find a Trivers-Willard
455 effect. In one study, for example, using years of
456 education as a proxy for parental investment,
457 Rosemary HopcroftAu4 (2005) found that sons of
458 high-status men attained more years of education
459 than daughters, whereas daughters of low-status
460 men reached higher education levels than sons.
461 She also found that high-status men sire more
462 sons. Future studies are needed to determine
463 whether the hypothesized Trivers-Willard effects
464 are found among different populations of humans.
465 Gaulin and Robbins (1991) found support for the
466 Trivers-Willard hypothesis in their study of North
467 American women. However, additional tests of
468 this hypothesis have produced mixed results, par-
469 ticularly with human samples. In a study with
470 larger data sets, Keller et al. (2001) reported that
471their results did not replicate the findings of
472Gaulin and Robbins (1991). Based on the analysis
473of the results from their own and other previous
474studies, these authors conclude that Trivers-
475Willard effects are at best tiny in the contemporary
476United States, where resources are too abundant
477compared to the typical conditions of hominid
478evolution.
479Reproductive Variance Hypothesis
480An alternative account of differential investment
481in sons and daughters focuses on the reproductive
482variance of sons versus daughters. The concept of
483variance in reproductive payoffs has been pivotal
484in some recent evolutionary analyses of behavior
485(see, for example, Daly and Wilson 1997). The
486evolutionary logic for risk/variance sensitive
487strategies is that selection would favor a greater
488risk-proneness when risk avoidance promises not
489fitness but reproductive failure. Universally and
490throughout hominid evolution, men have a higher
491reproductive variance than women. That is,
492women tend to consistently produce a few chil-
493dren whereas men tend to have either a lot of
494offspring or zero offspring. From a Darwinian
495perspective, risks can be viewed as variance in
496reproductive fitness in terms of the number of
497offspring one has (Wang 2002). Thus, parental
498investment in daughters versus sons is like a
499risky choice between a safer bet and a gamble
500(a more probabilistic bet), respectively.
501Following this line of thinking, parents from a
502contemporary US population whose wealth con-
503ditions are not clearly diversified tend to have
504comparable parental expectations (aspirations)
505for their children’sfinancial and reproductive
506prospects. Thus, what affects their differential
507investment in sons or daughters would be their
508psychologically perceived relative wealth com-
509pared to their neighbors instead of absolute
510wealth. When the perceived relative wealth is
511lower, the distance to parental expectations
512would be higher. Thus, sons would be favored
513since a higher variance in sons’financial and
514reproductive success would increase the chance
515of reaching the parental expectations or goals.
516Conversely, daughters would be favored by par-
517ents who have a higher perceived wealth.
6 Parental Investment Theory (Middle-Level Theory in Evolutionary Psychology)
518 Therefore, a reversed Trivers-Willard effect is
519 likely to be found in a less stratified population
520 as a result of the difference in perceived relative
521 wealth. In contrast, a Trivers-Willard’s effect
522 would be more likely to be found in a population
523 with a stratified wealth structure. For instance, if a
524 wealthy parent has a much higher than average
525 expectation for his or her children’sfinancial and
526 reproductive success, sons would be favored, thus
527 a Trivers-Willard’s effect.
528 In a field study conducted in villages in north-
529 west China, Wang (2002) found a reversed
530 Trivers-Willard’s effect when perceived wealth
531 condition (rated on a 1–9 scale against families
532 in the same local area) was taken into account.
533 The interbirth interval as a measure of parental
534 investment was significantly longer after having a
535 son than after having a daughter for the families
536 with lower perceived wealth but not the families
537 with higher perceived wealth.
538 In another field study conducted in a Midwest
539 U.S. rural community where the wealth structure
540 was also unstratified, Wang (2007) found that
541 actual household income affected overall parental
542 investment in a child irrespective of the sex of the
543 child, using breastfeeding and interbirth interval
544 as the measurements of the degree of parental
545 investment. In contrast, perceived wealth by par-
546 ents relative to their neighborhood households
547 affected differential investment in sons versus
548 daughters. Independent of real income of the fam-
549 ily, IBI was longer if the parents’perceived family
550 wealth was lower. This effect of perceived wealth
551 on IBI was evident both for daughters and sons.
552 As predicted from sex-specific reproductive vari-
553 ance, a differential breastfeeding pattern in sons
554 and daughters emerged. In the families of higher
555 perceived wealth, the average percent of daugh-
556 ters being breast fed was significantly higher than
557 that of sons (67 % vs. 36 %). In contrast, in the
558 families of lower perceived wealth, the percent of
559 daughters being breast fed decreased to 52 % and
560 the percent of sons being breast fed increased to
561 49 % percent.
562 The results from these two studies suggest that
563 investing in sons is riskier than investing in
564 daughters. In terms of parental economics, sons
565 can be viewed as a riskier prospect than daughters
566since men, on average, have a higher variance in
567wealth and reproduction than women.
568Conclusion
569Parental investment theories in evolutionary biol-
570ogy and evolutionary psychology have provided
571overarching frameworks and testable mechanisms
572that lead to novel predictions about parent-
573offspring relationships and investment. Fisher’s
574model focuses on the natural selection mechanism
575that maintains the 1:1 equilibrium in sex ratio and
576how parental investment covaries with the
577dynamic of the equilibrium. Hamilton’s inclusive
578fitness model reveals the production rules for the
579evolution of cooperation between kin, including
580boundary conditions for parental investment.
581Trivers’model identifies a link between parental
582behavior and sexual selection, and sex-specific
583differences in reproductive resource provision
584before mating and post-reproduction. Since that
585parental investment is costly for the parents and
586that the genetic relatedness of a child to either of
587the parents is 50 %, the ideal resource allocation
588for a child does not coincide with the ideal
589resource allocation for the parents. Thus, parent-
590offspring conflict is inevitable and ubiquitous.
591Parent-offspring conflict theory thus provides
592unique predictions for parental investment related
593behaviors. The r/K selection model and life-
594history theory view parental investment as an
595adaptation to environmental harshness and
596unpredictability regarding mortality and mobility.
597Evolutionary analyses also help to understand
598differential parental investment. The concept of
599paternity uncertainty is used to account for one
600type of differential parental investment that
601mothers invest more in offspring than fathers.
602The Travers-Willard hypothesis predicts another
603type of differential investment in sons versus
604daughters by the parents, contingent upon the
605wealth condition of the parents. An evolutionary
606analysis based upon reproductive variance in sons
607versus daughters offers an alternative mechanism
608for understanding differential parental
609investment.
Parental Investment Theory (Middle-Level Theory in Evolutionary Psychology) 7
610 Cross-References
611 ▶Evolution of Cooperation
612 ▶Genetic Relatedness Affects Aid to Kin
613 ▶Hamilton’s Rule and Theoretical Implications
614 ▶Kin Selection
615 ▶Life History Theory
616 ▶Operational Sex Ratio
617 ▶Parent-Offspring Conflict (Trivers)
618 ▶Paternity Uncertainty
619 ▶Reproductive Strategy
620 ▶Reproductive Value; Sex Ratio
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8 Parental Investment Theory (Middle-Level Theory in Evolutionary Psychology)
