Infant resuscitation is associated with an increased risk of left-handedness.
ABSTRACT The etiology of left lateral preference is not well understood, but some studies have suggested that it can be caused by complications at birth. The authors used data from the Child Health and Development Study, a large prospective study of pregnancy and child development conducted 1959-1966 in the San Francisco Bay Area of California, to examine the association between specific birth stressors and hand and foot preference. The study population consisted of 6,968 5-year-olds with no severe congenital abnormalities, and the authors controlled for potential demographic confounders and familial left-handedness. Infants who required resuscitation after delivery or who were twins or triplets were about twice as likely to demonstrate left hand preference at age 5 years (odds ratio (OR) = 1.8, 95% confidence interval (CI) 1.3-2.5, and OR = 2.2, 95% CI 1.2-4.0, respectively). Left-footedness was also significantly associated with the same stressors. No other individual stresses were significantly associated with left-lateral preference, and a composite measure indicated only a weak association. Although males, blacks, and those with left-handed siblings are more likely to show left lateral preference, these variables do not confound the association between birth stress and left lateral preference. These results indicate that specific types of birth stress are strongly associated with left hand and foot preference; however, much of the left laterality in non-clinical populations remains unexplained.
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ABSTRACT: The origins of human handedness remain unknown. Genetic theories of handedness have received much attention, but some twin studies suggest modest, perhaps negligible genetic effects on handedness. A related question concerning handedness is whether twins have higher rates of left-handedness than do singletons. We studied handedness, with information on forced right-handedness, in a sample of 30,161 subjects aged 18-69 from a questionnaire survey of the older Finnish Twin Cohort. Left-handedness was found to be more common in twins (8.1%) and triplets (7.1%) than in singletons (5.8%), whereas ambidextrousness was more common in triplets (6.4%) than in twins (3.4%) and singletons (3.5%). As in many other studies, males were more likely to be left-handed. Ambidextrous subjects were more likely to become right-handed writers even if not forced to use their right hand. We fit maximum likelihood models to our twin data to estimate the contribution of additive genetic, common environment and unique environmental effects to hand preference. Results, depending on the model, indicate that unique environmental effects account for most observed variance in handedness, both in childhood (92-100%) and adulthood (74-86%). When forced right-handedness was taken into account, estimates of familial effects increased. Concordance for left-handedness in twins is rare, and accordingly, very large samples are needed to detect the familial effects. Our results show that forced-handedness can have an effect on estimates of genetic effects.Neuropsychologia 05/2009; 47(5):1294-301. · 3.48 Impact Factor
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ABSTRACT: Since prehistoric times, left-handed individuals have been ubiquitous in human populations, exhibiting geographical frequency variations. Evolutionary explanations have been proposed for the persistence of the handedness polymorphism. Left-handedness could be favoured by negative frequency-dependent selection. Data have suggested that left-handedness, as the rare hand preference, could represent an important strategic advantage in fighting interactions. However, the fact that left-handedness occurs at a low frequency indicates that some evolutionary costs could be associated with left-handedness. Overall, the evolutionary dynamics of this polymorphism are not fully understood. Here, we review the abundant literature available regarding the possible mechanisms and consequences of left-handedness. We point out that hand preference is heritable, and report how hand preference is influenced by genetic, hormonal, developmental and cultural factors. We review the available information on potential fitness costs and benefits acting as selective forces on the proportion of left-handers. Thus, evolutionary perspectives on the persistence of this polymorphism in humans are gathered for the first time, highlighting the necessity for an assessment of fitness differences between right- and left-handers.Philosophical Transactions of The Royal Society B Biological Sciences 01/2009; 364(1519):881-94. · 6.23 Impact Factor
Article: Understanding left-handedness.[show abstract] [hide abstract]
ABSTRACT: The human cerebrum is asymmetrical, consisting of two hemispheres with differing functions. Recent epidemiological and neurobiological research has shed new light on the development of the cerebral lateralization of motor processes, including handedness. In this article, we present these findings from a medical perspective. We selectively searched the PubMed online database for articles including the terms "handedness," "left handedness," "right handedness," and "cerebral lateralization." Highly ranked and commonly cited articles were included in our analysis. The emergence of handedness has been explained by physiological and pathological models. Handedness arose early in evolution and has probably been constitutive for the development of higher cognitive functions. For instance, handedness may have provided the basis for the development of speech and fine motor skills, both of which have played a critical role in the evolution of mankind. The disadvantages of certain types of handedness are discussed, as some cases seem to be associated with disease. The consideration of handedness from the epidemiological, neurobiological, and medical points of view provides insight into cerebral lateralization.12/2011; 108(50):849-53. · 3.54 Impact Factor