Occasionally a sentence will change your life. In my case, it was when the doctor looked up from the sonogram screen and said, "Congratulations. You're having twins." After the shock wore off, I realized that twins are a gift to a biological psychologist. I was going to experience the nature-nurture issue up close and personal.
Our son Adam and his younger sisters Betsy and Katie (fraternal twins) were raised in the same home, exposed to the same foods, went to the same schools, and had the same family pets. All three turned out great. In many ways they are similar, but in some ways they are very different. Oddly, Adam is the only one with a Southern accent. He and Katie became extreme whitewater kayakers. Betsy, on the other hand, had no interest in running waterfalls in the middle of winter.
Our children also differ in their relationships with other species. Adam is the only real animal person. He and his wife are devoted to their two cats, and for ethical reasons he refuses to take his son to the zoo. Katie is not particularly concerned with animal welfare, but she has not eaten meat since she was 12. Her twin sister, on the other hand, has scarfed down crickets in Mexico, worms in Thailand, and (gasp!) whale meat in Japan. All three were exposed to snakes early on, when I maintained a reptile behavior lab, but in recent years Katie has developed a mild snake phobia.
Why Are Children in the Same Family So Different?
The patterns of similarities and differences seen in my children are common; you probably see them in your own family. Why are children in the same family so different from one another? This question was the title of a classic 1987 article by behavior geneticists Robert Plomin and Denise Daniels. Behavior geneticists love twins. The reason is that identical twins develop from the same fertilized egg and share 100 percent of their genes, while fraternal twins develop from separate fertilized eggs, and, like non-twin siblings, have 50 percent of their genes in common.
If genes play a role in the development of psychological traits, pairs of identical twins should be more similar than pairs of fraternal twins. Differences in the similarities of identical and fraternal twins can be used to calculate a statistic called heritability. A high heritability indicates that most individual differences in a trait are due to genes, whereas low heritability means that genes have little influence on behavioral differences. For example, genes account for about 99 percent of differences in eye color, 0 percent of differences in religious denominations, and 34 percent of differences in the frequency that women have orgasms during sex (see this article).
Studying Vets with Pets
You may be asking yourself, "What does twin research have to do with human-animal interactions?" Studies have found that kids who were raised with pets tend to keep pets as adults. Further, adults tend to stick with the type of pet they grew up with. These results, however, could be due to either genes or early experience. That's where twin studies come in. I am happy to report that, for the first time, a group of twin researchers has made a stab at untangling the roles of heredity and environment in our relationships with companion animals.
The research, which will appear in the December issue of the journal Anthrozoos, was conducted by a team lead by Dr. Kristen Jacobson of the University of Chicago.1 The subjects were part of the Vietnam Era Twin Study of Aging, an ongoing study of over 1,000 male twins who served in the military between 1965 and 1975. Roughly equal numbers of identical and fraternal twins participated in the research. Every couple of years, the men spend several days undergoing physical exams and filling out questionnaires. In a recent session, they were asked, "During the past 30 days, how often did you play with pets?" Bingo! Kristen's team realized that this question could be used to assess the role of genes in human-pet interactions.
The research team found that the identical twins pairs were more similar than the fraternal twins in whether they had played with pets. After crunching the numbers, Kristen determined that about 35 percent of differences in whether the participants played with pets was inherited. (Interestingly, genes have the same degree of influence on differences in the percent of time women experience orgasm during sex.) So it appears that a throw of the genetic dice plays a substantial role in whether men are "animal people" (and whether women regularly have orgasms).
Was I surprised by these findings? No. After all, genes influence a wide array of human traits. But I was surprised by what Kristen's group discovered about the effects of family environment on human-pet interactions.
Factors like living in the same house, eating the same food, and playing with the same pets constitute what behavior geneticists call shared environment. These factors tend to make siblings alike. In contrast, idiosyncratic events that kids experience, like having different kindergarten teachers or being bitten by a dog, tend to make siblings different from each other. These are called non-shared environment.
So what is more important in how kids turn out: shared environment or non-shared environment? You would think that being raised in the same home by the same parents with the same companion animals would make siblings alike when it comes to playing with pets. But you would be wrong. Indeed, the big surprise of the Vietnam Era Twin Study was that shared environment had virtually no influence on the frequency that the vets played with pets. (By the way, shared environment also plays no role in the percent of time female twins have orgasms during sex.)
So when it comes to interacting with companion animals (and female orgasm), non-shared environment and random factors are considerably more important than genes. In Kristen's study, they explained a whopping 70 percent of differences in playing with pets. These results suggest that being raised in "pet-friendly" family has surprisingly little impact on how people interact with companion animals when they grow up. Go figure.
This initial foray into the behavior genetics of human-animal interactions produced some fascinating results. First, the results are consistent with other studies showing that environmental factors tend to make children in the same family different rather than similar. Kristen's findings also lead to a host of other questions. For example, are genetic differences in playing with pets rooted in differences in love for animals or unrelated factors? These might include general health and fitness, playfulness, or sensation seeking. And what specific early experiences produce differences in attachment to pets in adults? Finally, the results suggest that heredity might explain why some people derive health benefits from living with pets while other do not.
While I cringe when I say it, "More research is needed."
1. Jacobson, K., Hoffman, C., Vasilopoulos, T., Kremen, W.S., Panizzon, M. S., Grant, M.D., Lyons, M. L., & Franz, C. E. (in press). Genetic and environmental influences on individual differences in frequency of play with pets among middle-aged men: A behavioral genetic analysis. Anthrozoos.
Hal Herzog is Professor of Psychology at Western Carolina University and the author of Some We Love, Some We Hate, Some We Eat: Why It's So Hard to Think Straight About Animals.