The debate about the influence of nature and nurture (or heritage and environment) on human development is one of the oldest and most controversial, both within and outside of psychology. It is related to some of the most fundamental questions that humans ask about themselves. How do we become the way we are? What makes us develop the way we do? These and other similar questions have been raised throughout this work concerning a wide range of topics: perceptual abilities, language acquisition, aggression, bonding, gender development, intelligence, schizophrenia, and depression. In some of these examples (language and perception), the focus of the debate is in a capacity shared by all humans, while in others (intelligence and schizophrenia) the emphasis has been placed on individual differences.
Nature Vs. Nurture
The intricate interaction of genetics and the environment has been described as follows: Genes do not determine complex traits on their own. Instead, genes and the environment interact with each other in a dynamic process. (1).
Nature
Within genetics (the science of inheritance), “nature” refers to what is commonly considered to be what is inherited, that is, the differences in genetic material (chromosomes and genes) transmitted from one generation to another (from parents to children).
So, what are genes? Genes are the basic unit of genetic transmission and consist of large molecules of deoxyribonucleic acid (DNA), where thymine binds to adenine, and cytokine binds to guanine. They are highly sophisticated chemical chains that form a double helix structure similar to a ladder.
More generally, genes do not cause behavior. Instead, they are chemical structures that have chemical effects on the body and, thus, influence behavior through their effects on the body’s response to the environment. Therefore, there are no “for” genes, let’s say neurosis (N), although there is abundant evidence that N has some genetic influence (such as inheriting a nervous system that is particularly sensitive to stress). Also, there are no genes “for” alcoholism, but genetic factors can, in a way, affect the body’s sensitivity to alcohol.
Claridge and Davis explain this point as follows: Genes encode very precise and microscopic fragments of biological material (proteins) that are physically and conceptually very distant from the complex behavioral and psychological characteristics in which they are supposed or must influence. But it is unlikely that there are genes or groups of genes “for” impulsivity, preference for homosexual relationships, religiosity, anxiety or even severe mental disorders, such as schizophrenia. The route from genes to behavior is undoubtedly much more tortuous than that and, for some given characteristic, it has to do with a reciprocal action between genes and environmental factors. (2)
Nurture
When the term “environment” is used in a psychological concept, it usually refers to all those postnatal influences that are outside or external to the individual’s body. Among them are other people, opportunities for intellectual stimulation and circumstances of the individual’s life. It is implicitly considered that these influences have an impact on a passive individual, who is molded by them.
When the environment is analyzed as a set of potential influences that have an impact on the individual, it is often broken down into factors such as overcrowding, poverty, socioeconomic status, family breakdown, marital discord and so on. For example, in studies on intelligence, children are often compared to these environmental factors, so it is typical for those from lower socioeconomic backgrounds to have moderate intellectual content than those from groups with higher socioeconomic status.
When families are compared in this way, it is assumed that children from the same family will have been affected similarly or equally by these environmental factors; however, in most of the characteristics, most children within the same family are not very similar. They are often extremely varied in personality, abilities and psychological disorders. This observation is most noticeable when two adopted children grow up in the same family. In general, they are no more than two people randomly chosen from the general population. This substantial variation within the family does not mean that the family environment is not essential. Instead, as indicated by Plomin (1996), environmental influences on development are shared on an individual basis by individual rather than on a family basis by family. (3)
In other words, the differences between children who grow up together are exactly what they would expect to find because the non-shared environment has more influence on development than the shared environment and the children within the same family have different experiences. For example, Dunn and Plomin found that the different ways in which parents respond to their various children are likely to have much more significant influence than the general characteristics of the family. Therefore, it is possible that it matters little if the children are raised in a home that is less loving or more punitive than the average, while it may be of considerable importance that a child receives less affection or more punishment than his siblings.(4)
Relation Between Nature-Nurture
As we have seen, the concept of gene-environment correlation is related to that of non-shared environments, which helps to explain the way in which the environment exerts its influence. Another way to consider its impact is to identify examples of gene-environment interactions.
In a genetic sense, phenylketonuria is a simple characteristic; it is a physical disorder produced by the inheritance of a single recessive gene of each of the parents. In it, the process by which the body provides typically the amino acid phenylalanine hydroxylase, which converts phenylalanine (a substance found in many foods, particularly dairy foods) into tyrosine, fails and phenylalanine builds up in the blood reducing blood levels of other amino acids. Consequently, the developing nervous system is deprived of the essential nutrients, which leads to severe mental retardation and, without intervention, to death. (5)
The relationship between what the child inherits (the two phenylketonuria genes, the genotype) and the signs and symptoms of the disease itself (high levels of phenylalanine in the blood and mental retardation, the phenotype) seems to be flat, direct and unavoidable: Given the genotype, the phenotype will occur. However, a routine blood test shortly after birth detects the presence of phenylketonuria genes, after which the affected baby is placed on a diet low in phenylalanine. This prevents the development of the disease. In other words, an environmental intervention will stop the phenotype from occurring.
According to Jones: If there is no one-to-one relationship between the genotype and the phenotype in the case of phenylketonuria, it is highly probable that there will be an even more complicated interaction in the case of intelligence, specific mental disorders, the personality, and others. One of these examples is the cumulative deficit. Another concept is facilitation. (6)
According to Horowitz, a facilitating environment is one in which the child has parents who love and respond to him and where he receives a fertile stimulation disposition. When the different levels of facilitation are combined with the vulnerabilities and susceptibilities initials of a child, an interaction effect is presented. For example, an elastic child (one with many protective factors and few vulnerabilities) can have a good enough result in a weak environment. In the same way, a non-elastic child can have quite good results in a highly facilitating environment. Only the non-elastic child who is in a deprived environment will have poor results.
Conclusion
Not all biological influences on development are genetic: some are critical aspects of the intrauterine environment that are sometimes confused with genetic influences. Therefore, “biological” does not mean “genetic” and, equally important, “nurture” can refer to biological, psychological, social and cultural influences. People also create their own environments and, for most of the characteristics, the shared environment seems to have little impact on development compared to the non-shared environment. The concept of non-shared environments helps explain how the environment influences development. Two ways in which non-shared environments arise are the gene-environment correlations and the gene-environment interactions. Phenylketonuria illustrates the absence of a one-to-one relationship between genotype and phenotype. Human traits are determined both by nature and by parenting. Where it is possible that researchers still disagree is regarding the relative contributions of each of them and on the specific genetic and environmental mechanisms that are involved.