The phrase ‘nature versus nurture’ is derived from early studies on the effects of parenting on childhood development. Researchers sought to determine the relative contributions of an individual’s innate qualities, determined by one’s genes (nature), and parenting or personal experiences (nurture) on the psychological and behavioral traits of children. If a child shows aggressive behavior, was he or she genetically ‘programmed’ to behave in such a way, or is it the product of his or her upbringing or environment? Consider if one of the child’s parents is also aggressive. Did the child acquire this behavioral trait through exposure to his or her parent's behavior or through inheritance?
No concept is as pervasive in the study of health and disease as distinguishing the effects of internal and external stimuli on bodily function. Since Claude Bernard elegantly introduced the idea, it has not only formed the basis of modern physiology but has also helped us understand numerous pathological states in terms of the interaction between inherited and environmental factors.
Over the years, the controversy has extended beyond childhood behavior to intelligence, sexual preference, and the propensity for certain diseases (see ''The Origin of Intelligence'' and ''Can You Raise Your Kids Gay?''). Despite being heavily researched, at least two problems make the nature versus nature debate a major challenge facing modern biology. The diseases and traits being investigated, particularly those to do with the brain, are themselves complex and often hard to characterize. Moreover, as our understanding of biology progresses, separating the consequences of intrinsic and extrinsic factors on a certain physiological or pathological state becomes increasingly difficult (see here).
Shedding Light on the Issue Using Twin Studies
Separating the effects of genes and environment on childhood development and the pathogenesis of diseases can be achieved by performing adoption and twin studies. Francis Galton first proposed this approach in 1875. It gained impetus at the beginning of the 20th century when Gregor Mendel’s insights into the mechanism of heredity became widely known. Identifying differences in specific traits by studying monozygotic twins, who share identical genetic information, over portions of their lives gives us insights into the contribution of the environment in developing these characteristics.
In terms of psychological traits, separated twins usually grow up to be very similar even when brought up in substantially different environments. In the landmark Minnesota Twin Study, which began in 1979, researchers studied more than 100 sets of twins or triplets that had been separated in infancy and raised apart from one another. They found that genetics can explain up to 70% of the variability in personality, intelligence, and temperament between the twins [1].
Implications Beyond Medicine
Investigating the relative contributions of innate and acquired factors in human psychology and health can have far-reaching consequences. Not surprisingly, the nature versus nurture debate has also made its way to the courtrooms. Many experts believe that criminality, for example, is a trait that is predominantly inherited. Thus, defense lawyers sometimes argue (with varying degrees of success) that, in certain cases, people accused of committing crimes cannot be held responsible for their actions because they cannot be held accountable for their DNA.
The discovery of rare mutations that strongly predispose to aggression, such as that of the monoamine oxidase type A gene, has helped encourage the acceptance of such legal arguments [2] (see also ''Tracing the Roots of Aggression"). Biologists and physicians often oppose legal battles that attempt to make use of such a defense. They believe that the public often poorly understands the link between genes and behavior, which is a complex issue.
Is the Debate Obsolete?
Over the past few decades, we have made some astounding discoveries regarding how our genetic material is controlled. We now know that DNA is not the rigid, unchanging blueprint of our entire lives that it was once thought to be. Gene expression is a flexible (yet tightly regulated) process that is modulated continuously in health and disease.
Epigenetics (meaning ‘in addition to’ genetics) is the field of biology that deals with the alterations in gene expression that occur in the absence of changes to the DNA sequence. These changes can persist over long periods of time and, perhaps most interestingly, can be inherited from one generation to the other. The signals that trigger epigenetic changes can come from within the organism itself or from the surroundings (see "Lamarck's Last Laugh" on p.4).
The more we learn about epigenetics, the smaller the distinction between nature and nurture becomes. Consider an example. Exposing an individual to stress can alter the expression of proteins involved in the pathogenesis of mood disorders [3]. This altered expression can persist not only throughout the individual’s lifetime, but can also be transmitted to his or her offspring. If one of this individual’s children eventually develops depression, is the contribution of the exposure of the child’s parent to stress inherited or environmental? The simple answer is both. It thus comes as no surprise that, nowadays, many experts consider the debate obsolete.
Although we should not insist on drawing a line where boundaries are becoming less and less clear, making a distinction between the effects of nature and nurture aids our understanding of complex biological processes.
[1] Bouchard TJ Jr et al, Science, 1990
[2] Brunner, Nelen et al, Am J Hum Genet, 1993
[3] Murgatroyd, Nat Neurosci, 2009
by Ahmed Khalil
This article originally appeared in CNS Volume 7, Issue 3, Nature vs Nurture
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