Maybe you were raised in a house full of prowling, meowing, under-bed lurking cats, and find that, as a mature, taxpaying adult, you simply can't live without a cat or two or six or, fine, a baker's dozen, just so that if the cats ever have to vote on something, they won't experience democratic gridlock and just move to appoint a tyrant cat. Good for you! Animals make for hilarious companions, but your affinity for them probably has little to do with your cat-saturated upbringing.
In a Psychology Today article that ruminates about why siblings (fraternal twins being an especially revealing example) raised in the same animal-friendly household do not necessarily cling to animals throughout their lives, Hal Herzog offers up the findings from the ongoing Vietnam Era Twin Study of Aging, which has managed to figure out that animal lovers (at least male animal lovers) are born, not made, and that a whole childhood of pet puppies does not a dog lover make. A team helmed by the University of Chicago's Dr. Kristen Jacobson conducted the research, which focused on more than 1,000 male twins (roughly equal parts fraternal and identical) who had served in the military between 1965 and 1975.
The difference between fraternal and identical twins is important in determining the heritability of being an animal lover, since identical twins develop in the same fertilized egg and share 100 percent of their genes, while fraternal twins develop in separate eggs and share (like regular siblings) 50 percent of their genes. As part of the twin study, participants were asked a new round of questions every few years, the most recent of which included the question, "During the past 30 days, how often did you play with your pets?" That question turned out to be a rabbit hole of determining the role genes played in whether someone was an animal-friendly person or a crank who says things like, "Dogs need to sleep outside," or "But I'm allergic to salamanders!"
Unsurprisingly, identical twins had more similar pet-romping habits than their discrete egg counterparts. Jacobson determined, after going over the numbers, that about 35 percent of differences in whether participants lavished attention on their pets was inherited, the same difference, as Herzog points out four whole times throughout his article, in the percent of time women experience orgasm during sexy times (orgasm-powers, too, it seems, are inherited gifts).
Did that surprise Herzog? Not the slightest — he's a bona fide Ph.D., after all and, what's more, he has kids of his own, one of which loves animals while the other two (fraternal twins) don't care for them very much (one even ate whale meat, which is the moral equivalent of spray-painting a panda bear black, releasing it on the Appalachian Trail and leaving it to an unglamorous, frustrating life of trash-diving). Jacobson's research did, however, uncover this interesting observation on the importance of shared environment (home life, for example) or non-shared environment (random, "idiosyncratic" events) on the different ways siblings respond to animals:
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 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.)
It's weird how often that orgasm factoid keeps popping up, but we have to focus — animals, and the liking thereof. Non-shared environmental factors — all those times that one sibling was chased down the driveway by a Rottweiler while another spooned the family Golden Retriever while partaking in a Flash Gordon marathon — are considerably more important than genes, accounting for 70 percent of the differences in pet-playing responses. This suggests that being an animal lover has less to do with how peaceable your childhood menagerie of pets was and way more to do with whether a pack of tabbies menaced you for milk money one day after you missed the school bus.
Are You an Animal Person? It Could Be In Your Genes [Psychology Today]