Brennan: The Transmission of Affect

Brennan laments the increasing popularity of genetic explanations for social behavior. She asserts that it is “not genes that determine social life; it is the socially induced affect that changes our biology” (1-2). While Brennan is surely right in some ways, it’s possible that she overlooks some aspects of the genetic make-up of humans. It is widely accepted across the various fields of genetic and environmental research that neither genes nor environment can be cited as the sole explanation for any human phenomenon. Rather, it is at the intersection of these two, called gene-environment correlations, that answers are likely to be found.

Two important terms in any discussion of genetics are genotype and phenotype. The genotype is the genetic endowment that an individual inherits upon conception—and is generally what people have in mind when they talk about something being “in your genes.” Phenotypes, on the other hand, are the way each individual’s genetic endowment (i.e., genotype) is expressed, and is largely dependent on the person’s environment. I’ve always thought of these as sort of a formula:

Genotype (inherited genetic potential) + Environmental Influences (home, nourishment, emotions) 

= Phenotype (the way an individual’s traits manifest in observable or measurable characteristics)

So, even if a “Billy” has all the right genes to be a 7-foot-tall basketball star, he may never bypass average height if he grows up in an environment that fails to provide him the nutrients required for healthy growth. Psychological researcher Keith Yeates demonstrated this phenomenon in terms of children's intelligence. At age two, the best predictor of a child's intelligence was his or her mother's IQ, and the home environment had little effect. At age four, the Mother's IQ and the home environment were equally important in predicting a child's IQ.

Eventually, however, differences in children's home environments influence children's IQs beyond the effect of their mother's IQ. Other studies have found that a child's IQ increases when he or she is move from a low to a high intellectually stimulating home, and that genetic differences related to intellectual capacity are only relevant in high SES environments. In low SES areas, an individual with a genetic predisposition for intellectual endeavors may not have access to the resources and stimulation required for him to realize that potential. Furthermore, the quality of a child's day-care facility can predict his or her verbal intelligence score.

The same gene-environment correlations have been observed in studies of temperament and personality. Temperament is an individual's tendency to respond in typical ways, such as in response to social or emotional situations. Anyone who has held more than one baby would surely agree that babies show an impressive range of temperament. Twin-studies demonstrate a genetic contribution to an individual's temperament. The temperaments of identical twins were moderately correlated, suggesting that genetic endowment accounts for 50-60% of the variation in their temperaments. Fraternal twins' temperaments, on the other hand, had a correlation of zero.

Proponents of the concept of gene-environment believe that genes and environments are systematically related--people with different genes respond differently to different situations, and people with different genes encounter different situations. Researchers hypothesize three different types of gene-environment correlations: passive, evocative, and active. A passive gene-environmental correlation occur when a parent provides children with their genes, and are raised in an environment shaped by those genes. A paranoid parent, for example, gives a child his paranoid genes, and raises him in a paranoid environment. Evocative gene-environment correlations are seen when a child's genetic endowment (such as her happy temperament) affects her environment. A happy, giggly baby, for example, is likely to be showered with more kisses, smiles, and hugs than a baby with an uneasy temperament. The third type of correlation, active gene-environment correlations, occur when a person's genetic make-up influences the type of environments he chooses. If Tommy is social, he'll go to parties. If Sally is shy, she might prefer a quiet night in.

So, when Brennan asserts that it's "not genes that determine social life; it is the socially induced affect that changes our biology,” she seems to be focusing on one component of a highly interrelated set of factors. Socially induced affect does change our biology, but surely our biology affects the social affect we elicit and encounter.