Postnatal environment

Caldji, C., Liu, D., Sharma, S. et al. Development of individual differences in behavioral and endocrine responses to stress role of the postnatal environment. In Coping with the Environment Neural and Endocrine Mechanisms. Ed. McEwen, B. S. New York Oxford University Press, 2000, Vol. IV, pp271—292. 

Bjorksten, B. (1999). The intrauterine and postnatal environments. J. Allergy Clin. Immunol. 104,1119-1127. 

One reason is that identical twins have a common prenatal and postnatal environment. A second is that the incidence of abnormalities is increased by twinning itself. A third is that the incidence, to the extent that it is genetic, may depend on balancing selection, rather than mutation. 

My intention throughout the book is to make the case that the prenatal environment must be considered in any attempt to understand the origins of human health and disease, and of human behavior and intelligence, and in any resolution of the simplistic nature—nurture debate put to the public by the media. Nature is more than genetics, and nurture is a more complex proposition than what occurs in just the postnatal environment of parents and communities and nations. Environment starts at conception, and neglecting this fact as we have for too long has resulted in often tragic consequences for those born into our world and, by extension, for us all. 

The third set of variables lies in the postnatal environment. These include the physical environment (outdoors and in) and the social environment (family, peers, neighborhood, socioeconomic status, education, culture, and so on). Many of these variables affect the development of emotions, learning, language skills, and the processing of information by the brain—and can also affect gene expression via stress, infection, malnutrition, and toxins in air (for example, lead), water (for example, arsenic), and food (for example, mercury). Those effects that occur during early development can be extremely critical. 

What seems apparent from all these cases is that human sexuality is a good example of how genes, prenatal environment, and postnatal environment all contribute to behavior traits. As such, absolute genetic determinism of gender seems an untenable idea. Both genes and environment are involved—prenatal and postnatal environment—and how they re involved may differ in each case. 

Nor are genes and heredity the exclusive determinants of sexual orientation, which is instead derived from a complex of variables involving genes, prenatal environment, and postnatal environment. Four real empirical situations illustrate variations of this point. 

The second situation involves a set of monozygotic male triplets, age 21 years, in which two of the triplets are heterosexual and one homosexual.17 Again, we have identical inherited genomes with different developed sexual orientations. In both situations, other factors—pre- and postnatal environment— must somehow come into play. 

Definitely not. The similar environments in these studies is the postnatal environment. The prenatal environment is never considered in the analysis. For twins reared in similar postnatal environments, we don t know if the higher twin-pair IQ correlations for MZ twins are due to identity of genes or similarity of fetal environments. It is certainly possible that the fetal environments (including reactions to impacts) of identical twins are always more similar than the fetal environments of fraternal twins. We don t know enough yet about human twin biology to discount that possibility. 

At 2 years of age the twins may have had the same IQ due to genes or to the fact that they came out of the same uterus or due to some combination of variables—we don t know. What we do know is that every foster-home twin in this thought experiment has evidently experienced a large IQ deficit produced by the change in postnatal environment, the one that is probably the most important developmentally. 

None the less, infants are clearly capable of olfectory learning and memory formation beginning within the first hour after birth, and those memories may be retained over an interval of at least several days. Neurophysiological processes that occur naturally during parturition prepare the newborn in nt to become familiarized rapidly with salient odors in the postnatal environment. Neonates may be particularly sensitive to stimulus input during a brief period immediately after birth as a result of elevated NE levels and brain arousal at that time. 

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