This paper reviews the earliest documented manual and postural asymmetries, in the fetus and during the first months of life. I attempt to analyze which genetic and/or environmental factors are likely to trigger each one, as well as its consequences for the other ones. I conclude that right-handedness is prevalent in all cultures because an intrinsic tendency toward right-handedness has many occasions to be reinforced, from the uterine to the perinatal environment and from the familial to the cultural environment. Finally, the combination of potential genetic factors-direct (motoric) or indirect (postural)-with varied biological and cultural environmental influences over various periods during development may explain the high variability of handedness in typical populations (as long as hand preference is not equated with the hand used for writing).

The vast majority of humans are right-handed, but how and when this bias emerges during human ontogenesis is still unclear. We propose an approach that explains postnatal handedness starting from 18 gestational weeks using a kinematic analysis of different fetal arm movements recorded during ultrasonography. Based on the hand dominance reported postnatally at age 9, the fetuses were classified as right-handed (86%) or left-handed, in line with population data. We revealed that both right-handed and left-handed fetuses were faster to reach to targets requiring greater precision (i.e., eye and mouth), with their dominant (vs. non-dominant) hand. By using either movement times or deceleration estimates, handedness can be inferred with a classification accuracy ranging from 89 to 100% from gestational week 18. The reliability of this inference hints to the yet unexplored potential of standard ultrasonography to advance our understanding of prenatal life.

Across time and place, right hand preference has been the norm, but what is the precise prevalence of left- and right-handedness? Frequency of left-handedness has shaped and underpinned different fields of research, from cognitive neuroscience to human evolution, but reliable distributional estimates are still lacking. While hundreds of empirical studies have assessed handedness, a large-scale, comprehensive review of the prevalence of handedness and the factors which moderate it, is currently missing. Here, we report five meta-analyses on hand preference for different manual tasks and show that left-handedness prevalence lies between 9.3% (using the most stringent criterion of left-handedness) to 18.1% (using the most lenient criterion of non-right-handedness), with the best overall estimate being 10.6% (10.4% when excluding studies assessing elite athletes’ handedness). Handedness variability depends on (a) study characteristics, namely year of publication and ways to measure and classify handedness, and (b) participant characteristics, namely sex and ancestry. Our analysis identifies the role of moderators which require taking into account in future studies on handedness and hemispheric asymmetries. We argue that the same evolutionary mechanisms should apply across geographical regions to maintain the roughly 1:10 ratio, while cultural factors, such as pressure against left-hand use, moderate the magnitude of the prevalence of left-handedness. Although handedness appears as a straightforward trait, there is no universal agreement on how to assess it. Therefore, we urge researchers to fully report study and participant characteristics as well as the detailed procedure by which handedness was assessed and make raw data publicly available.

This review summarizes the present evidence for a biological basis of functional brain asymmetry. Morphological asymmetries, despite long-standing knowledge, have only recently aroused general interest. The most striking asymmetries involve regions of the cerebral cortex located around the posterior end of the Sylvian fissures: mainly studied was the planum temporale, but others include the parietal operculum, inferior parietal lobule, and inferior frontal gyrus. In most instances, cortical areas proved to bear asymmetries favouring the left hemisphere. Architectonic studies also revealed asymmetrical features; especially area "Tpt", roughly similar to Wernicke's posterior language area, has been found up to 7 times larger on the left than on the right hemisphere. Similar asymmetries were discovered in fetal brains, as early as the 30th gestational week, as well as in the cerebral cortex of some apes. These data suggest that morphological asymmetries need not be the consequence of functional effects but rather a predetermined feature, probably widely spread throughout animal kingdom. An open question remains as to the functional significance of these asymmetries. In this regard, studies have dealt with possible correlations between morphological asymmetries (as assessed in vivo by cerebral neuro-imaging methods) and features of functional asymmetry, especially handedness and hemispheric dominance for language. Despite incomplete results, available data suggest a significant correlation, at least in dextrals. The exact nature of these relationships remains speculative. Knowledge about the contribution of genetic factors in determining cerebral dominance followed observations of familial clustering of functional asymmetries, especially sinistrality, as well as studies in mono- and dizygotic twins. However, a purely genetic model seems to be unable to account consistently for the data. Hypotheses emphasizing birth stress may be only exceptionally verified. Actually, current evidence points to a specific period in the fetal life, probably around the 6th gestational month, during which neurons in post-migrational stage set up their synaptic contacts. The final gyral pattern of the brain probably builds up during this period, as a consequence of a mechanism of competition for synapses among different cortical areas which regulates the amount of neuronal growth and death. Such a mechanism probably also accounts for the development of cortical asymmetries.(ABSTRACT TRUNCATED AT 400 WORDS)