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.
via Whisker Therapy