Nature vs. Nurture: Genetic and Environmental Influences on Human Behavior

Introduction and Background

For decades, scientists and philosophers of every sort have attempted to determine whether nature or nurture is the more important factor in human behavior. Theologians have also entered the debate, as questions about how to treat various socially influential behaviors — such as gender identification and homosexuality — make arriving at some conclusion regarding the genesis of human behavior increasingly compelling. The work to date on the subject seems to point in the direction of a duality, though one aspect of that duality may be worthy of slightly more weight. There appears to be a consensus across many fields of study that nature is more largely responsible for determining human characteristics than nurturing by agents of enculturation, but that culture does nonetheless exert a great influence on behavior.

Behavior is one of the most complex and interesting of human characteristics. Like many other characteristics — such as height or weight — behavior has come to be understood as reflecting a combination of influences, some genetic and others environmental. While it had for many years been assumed that behavior was essentially all learned, recent advances in a number of research techniques have allowed scientists new and provocative glimpses into the genetic basis of human behavior. Moreover, numerous disciplines — from anthropology to law to zoology — have contributed studies and data toward more reliably determining the relative contributions of genetics and environment to human life. It appears, especially in light of the discovery of various genetic markers for all sorts of human characteristics, that nature is more largely responsible for determining human characteristics than is nurturing by agents of socialization. However, the environment — our human cultures and the resulting enculturation of individuals — may be more greatly responsible for shaping an individual's specific behavior than are that person's genetics.

The following preliminary research explores some of the aspects of the nature versus nurture controversy. The complex interplay between genotype and phenotype are variables studied via findings across various disciplines to determine whether the genotype (the nature aspect) is more influential than the phenotype (the nurture aspect) in determining human behavior.

Gender Identity and the Nature–Nurture Debate

On the theory that no behavior is more emblematic of a person's broader behavioral profile than their sexual behavior — which arises from their sexual identity — a study of gender identification and the nature-nurture debate seems a logical starting point. It should be noted, however, that a case might be made that this is not entirely so, at least in the case of transgender individuals and those whose behavior includes gender-nonconforming expressions such as cross-dressing. An article by Hausman (2000) dissects the results of enculturation of — and often surgery upon — individuals born with indeterminate physical sexual characteristics. Hausman uses the case of twin sex reassignment to comment on the way gender operates in the nature versus nurture debate, particularly in terms of gender identity.

Hausman (2000, p. 115) cited research noting that "96 percent of intersex infants are 'made into girls' (Dreger 1999). One in 1,500 infants are born with genitalia so unusual that sex assignment into the standard categories of male and female is difficult, although one in 200 or 300 infants are referred to surgery because of 'somewhat problematic' genital configurations, such as hypospadias, a condition in which the urethra does not exit from the tip of the penis (Dreger 1999)."

While that had been standard medical procedure for years, a case involving a child who had lost his penis in the first year of life and was surgically changed into a girl was causing medicine to rethink its assumptions. After reaching the age of reason, the child "was living as a man." Some who studied the case concluded that it demonstrated that "gender identity is not malleable before a specific age, as Money had originally asserted, but that it is innate and based on chromosomal and hormonal sex factors" (Hausman, 2000, p. 115). Hausman (2000, p. 116) also noted that "the case in all its guises demonstrates that gender identity is the result of a process of self-naming that is embedded within the cultural milieu and influenced by its gender stories." These are contrary claims; however, they do substantiate the concept that nature is the prime causative factor of behavior, while culture also plays a role in influencing behavior. Based on this case, genotype appears to influence gross behavior (male or female identification), while phenotype influences the particularization of that behavior.

Hausman (2000, p. 117) attributes this particularization to "narrative." She notes that usually, "narratives make sense, literally by producing the patterns through which we learn to understand the world around us, patterns which become models for how 'sense' is made." To explain enculturation more broadly, Hausman further notes that "repetition of patterns is one way in which the sense of ideas as narratives (that is, as particular ordered tellings, as stories) is lost, and the information conveyed becomes instead a set of received ideas, facts without narrative history." She concludes that, regardless of physical makeup, "none of us is fully a 'boy' or a 'girl' until that identity is made for us by our family and community and embraced by us" (Hausman, 2000, p. 117).

Using narrative as Hausman does is not far from some modern anthropological methods of investigating the nature/nurture dichotomy. Noting that "cognitive anthropology is essentially an inferential study, since anthropologists infer informants' ways of thinking from linguistic expressions," Astuti (2001) attempted to describe how the Veso of Madagascar "reason about the distinction between birth and nurture, organism and person, mind and body."

Anthropological and Psychological Perspectives

Astuti (2001, p. 429) notes that the Veso "find the suggestion that birth parents have exclusive claims over their children morally problematic, and they express this view in a variety of ways. Thus, they seemingly do not 'see' resemblances between parents and their offspring." That does not mean, of course, that such resemblances do not exist, but it does indicate that society is partially responsible for the importance placed on genetics versus culture. The Veso seem to invoke what might appear to a cultural anthropologist studying traditional peoples as something close to magical thinking to explain why a baby might come to resemble people other than its parents: "For example, if a pregnant woman spends a lot of time talking to a friend, the friend is said to 'steal' some of the baby's facial traits; if she takes a strong dislike to someone, her baby will look like the disliked person" (Astuti, 2001, p. 429). Astuti concluded that this socialized parenthood; it could easily be argued that it demonstrates beyond doubt that nurture has a significant effect on the individual — not only on behavior but on physiognomy as well, though one would need to investigate gene donations among members of the group to be more definitive.

Because anthropological investigation depends on so many other fields — from art to linguistics to zoology — it is reasonable to look at psychology as well for insight. Moore, writing in The Psychological Record, posed the nature/nurture question as directly as possible, asking: (1) How is an organism's behavior functionally related to its environment? and (2) How do the organism's neural and hormonal systems mediate those functional relations? (Moore, 2002, p. 261+).

While Moore was particularly interested in the second question, he remarked on the first that there are apparently three factors governing the relation between environmental circumstances and an organism's behavior. Citing Catania and Harnad (1988), he proposed that "these relations exist at phylogenic, ontogenic, and cultural levels" (Moore, 2002, p. 261+).

Moore approached the nature versus nurture question at a foundational level, considering it in terms of species survival. He wrote: "Suppose that certain forms of responsiveness favor survival. Organisms that possess these forms survive for one reason or another… The forms of innate responsiveness contribute to a behavioral definition of a species, as a kind of behavioral phenotype" (2002, p. 261+). He regarded this as conditioned respondent behavior that occurs "when stimulus A, which initially did not elicit a response within a particular response system, comes to do so because of a close and consistent relation between stimulus A and stimulus B, which initially did elicit a response within the response system in question" (Moore, 2002, p. 261+). In short, he argues that a learning curve of some sort underlies the human behavior that allows the species to survive. This still does not address, however, whether there are differences within response behaviors that depend on genotype. While all humans might run from fire, might those of varying genotypes do so in different ways? And might different cultures eventually imprint the genotype with a particular form of behavior? While this question lies beyond the scope of this investigation, it, too, may elicit the sort of debate that has attended the nature/nurture question for decades.

Conclusion

The work to date on the subject seems to point in the direction of a duality, although one aspect of that duality may be worthy of slightly more weight. There is a consensus across many fields of study that nature is more largely responsible for determining human characteristics than nurturing by agents of enculturation, but that culture does have a great influence on behavior. The case studies and theoretical frameworks reviewed here — from gender identity reassignment and Veso cultural anthropology to behavioral psychology and the neuroscience of fear — consistently support the view that genotype establishes the broad parameters of human behavior, while phenotypic and cultural forces shape how that behavior is expressed in individuals and communities.