Genetics, Genomics, Genethics: Course Preview

Nature v. Nurture

by Dr. Claudia Englbrecht

This essay was developed for Week 2 of the AMNH online course Genetics, Genomics, Genethics, part of Seminars on Science, a program of online graduate-level professional development courses for K-12 educators. Explore more sample resources...

"Nature versus nurture" is a phrase used widely to express the long-standing scientific debate about whether we are mainly "made" by our genes or whether influences from the environment are more important in our construction. Today, we know that our nature (our genes) and our environment (which includes our development in the womb, nutrition, personal contacts, parents, friends, experiences, the climate, etc.) together make us individuals. We are the result of the subtle "cooperation" of genes and environment. This is why the terminology "nature versus nurture" is misleading. It is misleading because it is not "versus," but rather "and" or "together."

Venus and Serena Williams, two sisters, are considered world-class tennis players. Is their success a result of their similar genes, or evidence of their similar training? ©RowZTennis

There are probably only a few traits, such as hair color, eye color, and blood type that are determined solely by genes. Other traits, such as physical condition and health, are strongly influenced by the environment. The majority of our traits are influenced to a varying extent by our individual environments. This is obvious in language. The French speak French with a fluency no American can achieve.

But height is also determined by nurture; during the last century the overall height within populations increased in almost all countries that industrialized and improved both hygiene and nutrition. This trend is not related to changing genes. The genotype is all the specific gene versions an organism possesses. The phenotype is the detectable outward manifestation of a genotype and is also influenced by the environment (e.g., hair color). The genotypic composition of these populations did not change drastically during that time. This shows that organisms have great phenotypic plasticity (an observable change of a genetically controlled trait) for certain traits, i.e., genes produce a wide range of phenotypes across a range of environments.

The view that most of our traits, including personality, result from genes and environment was not always shared. Only a few decades ago, the prevalent opinion among behaviorists was that “the human is an empty being.” Almost everything in human behavior was explained by exogenous influences. Sound scientific studies and evolutionary theory have led to the view we have today about the interaction of genes and environment in the development of traits. Of course, scientists are not satisfied with the mere notion that genes and environment both affect our traits: They also want to understand the interaction more precisely; namely, how does the environment impact special traits? This is particularly interesting in the case of diseases, such as asthma or cancer.

Psychologists have a strong interest in determining the role of genes in the formation of our personality, our intelligence, and our social ability. Scientists are trying to assess the so-called heritability of a trait (not to be confused with heredity), or how much of a trait is a result of genes. Gaining that insight is also very important for understanding the causes of mental illnesses and how to treat them.

How does one properly measure the heritability or the impact of nurture on traits? It is not an easy task. Humans are genetically variable, and therefore comparisons between two individuals don’t always allow for inferences about the influence of nurture to be made. In addition, the measurement of intelligence or social ability is a rather subjective endeavor. In order to assess the environmental influence on traits, it is helpful to compare traits in genetically identical individuals.

The only naturally occurring genetically identical humans are identical twins, or triplets, quadruplets, etc. There is a specific discipline dedicated to twin studies. In such studies, a few approaches can be taken. Traits are measured in identical twins that grew up together in the same environment, and in identical twins that were raised apart in different environments. Important inferences can be made when comparing data from those studies with data from fraternal twins and siblings who grew up in the same family—siblings who shared the same environment at the same time, rather than siblings who were not exactly the same age and therefore didn’t share the exact same family experiences. Adopted children are included in these studies, too. The adopted children serve to determine which traits appear to be most affected by the presence of biological versus foster parents.

Intelligence

Twin studies showed that 85% of identical twins raised together and 70% of identical twins raised apart had the same IQ. This would imply that genes have a major impact on intelligence, but how does it work? The explanation for these results could partly be that the identical twins chose (as a result of their genes) the same social environments in their lives even when they were raised apart and therefore were similar in their IQs. A study of over 212 twin studies that examined the heritability of intelligence concluded that 85 percent of identical twins raised together, and 74 percent of identical twins raised apart, had the same IQ, compared with much lower percentages for siblings raised together (45 percent) and apart (24 percent) (Devlin et al., 1997). This implies that genes have a major impact on intelligence, but how do they work? One explanation for these results is that the identical twins chose (as a result of their genes) more similar social environments in their lives than siblings would, even when they were raised apart, and thus they had more similar IQs. A more recent MRI study provides another explanation, pointing to a genetic influence for the size of some of the regions of the brain involved in cognitive function (Thompson et al., 2001). Thus, twins with identical genes are likely to produce similar brain sizes.

It is still hotly debated whether intelligence, as measured by multiple-choice tests, is determined by nature, nurture, or both. ©AMNH

Aggression and Humor

According to family studies, the influence of family environment on the personality development of children is, perhaps surprisingly, relatively minor (except in cases of severe neglect and abuse). Environmental influences outside the family, such as school and friends, are often more important to the development of personality. However, these "outside influences" may have more to do with genes than it would seem at first. People might choose, or even create, environments to which they are genetically predisposed. For example, an aggressive person is more likely to find an environment in which arguments are likely to take place, or to create such an environment by starting fights, than a peace-loving person might. In these cases, the assessment of whether genes or environment is more important resembles the question of the chicken and the egg.

As for humor, a study concerned with the genetic basis of humor suggested, by comparing reactions to Gary Larson’s The Far Side cartoon, that upbringing has more influence than genes on the sense of humor (Cherkas et al., 2000). However, there did not appear to be a general humor factor underlying responses to the cartoons, and no effect of age was seen.

The way genes influence our personality may be by directing the development of the nervous and endocrine systems, and the body chemistry that rules our behavior. By comparing personality traits, it should be possible to find links to differences in body chemistry—and in fact, more and more of these links are being discovered.

Hopefully this short description of some of the ongoing research into the nature/nurture debate provides an idea of how complex the development of human traits is. It should be clear that, except for a few cases (such as some diseases), we are not solely determined by our genes. However, in the future, the danger of genetic testing lies in the fact that other people and institutions (e.g., insurance companies) will try to classify us, and determine our genes, so they can estimate our predisposition to disease or even to specific behaviors.

References:

  • Cherkas, L., F. Hochberg, A.J. MacGregor, H. Snieder and T.D. Spector. (2000). “Happy Families: A Twin Study of Humor.” Twin Research 3, 17-22.
  • Devlin, B., M. Daniels and K. Roeder. (1997). “The heritability of IQ.” Nature 388, 468-471.
  • Thompson, P., T.D. Cannon, K.L. Narr, T. Erp, V. Poutanen, M. Huttunen, J. Lönnqvist, C. Standertskjöld-Nordenstam, J. Kaprio, M. Khaledy, R. Dail, C.I. Zoumalan and A.W. Toga. (2001). “Genetic Influences on Brain Structure.” Nature Neuroscience 4 (12).

Online Resources: