There have been many studies demonstrating the health and psychological benefits of having dogs, but it’s difficult to disentangle possible genetic factors that could be influencing the results. It is possible that people’s genetics affect their likelihood of having dogs and that those same genes are influencing their decision to welcome dogs into their lives. Twin studies are an important tool for determining the role that genetics plays in all sorts of aspects of human life, and separating the role of our genes from the role of our environment on who we are as people.
The key to this technique is that there are two types of twins and these two types share different amounts of genes in common. Monozygotic (identical) twins share 100 percent of their DNA with each other but dizygotic (fraternal) twins share an average of 50 percent of their genes with each other. If they are raised together, both types of twins grow up in essentially the same environment.
A previous study showed that in a comparison of the propensity to engage in play with dogs, 37 percent of the variation could be attributed to genetic factors but only 10 percent was related to the environment in which people grew up. That shows a substantial contribution of genetics to play behavior with dogs.
The tendency to have dogs has never before been studied in twins, but thanks to the large twin registry in Sweden, scientists were able to explore it. In a new study published in the journal Nature (“Evidence of large genetic influences on dog ownership in the Swedish Twin Registry has implications for understanding domestication and health associations”), scientists attempted to estimate the heritability of dog ownership. The Swedish Twin Registry is the largest of its kind in the world, and for this research, 35,035 pairs of twins in the registry were studied, all of them born between 1926 and 1996.
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Dog ownership from 2001 through 2016 was investigated in this study. Swedish law requires all dogs to be registered with the Swedish Board of Agriculture and purebred dogs with certified pedigrees are registered with the Swedish Kennel Club, so the vast majority of dogs in that country can be accounted for.
Approximately 10 percent of the people in the study had dogs, with more women having them than men. Complex statistical analyses (typical for large studies like this) revealed a heritability for dog ownership in the range of 57 percent for females and to 51 percent for males. The shared environment accounted for much less—in the range of 0 to 6 percent.
There are two reasons that the results of this study are important scientifically. One, if there is genetic variation that affects dog ownership, it is possible that genetic variation was an important factor that affected humans’ ability to domesticate dogs and other animals. Two, whenever studies assert that there are health effects related to having a dog, it is important to consider that there may be genes that influence both health and the likelihood of having dogs. (This would be an example of pleiotropy. In genetics, the term “pleiotropic” refers to a gene that effects multiple and seemingly unrelated traits.) That could mean that having a dog is not causing health effects but rather that something is causing both a health effect and the tendency to have a dog.
This study does not provide information on which gene (or far more probably genes) influences the likelihood that people will have dogs. It does provide evidence of a strong genetic component to dog ownership. That fits with the feeling that many of us have that loving dogs is an intrinsic part of who we are. It’s wonderful to be able to say that a desire to have dogs is in our DNA and to mean it literally.