My skepticism of twin studies

I’ve always been skeptical about twin studies, and long before I had identical twins.

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This one claims that more than two-thirds of a child’s food avoidance is genetic. More specifically, the researchers studied parents of 37 monozygotic (identical) and 29 dizygotic (non-identical) twins. Parents completed questionnaires about their children’s food eating habits, and how their children are fed, etc. The researchers also measured the twins’ physical dimensions and body fat.

The study found that picky eating is strongly related in identical twins (r=.71) and not at all related in non-identical twins (r=-.01) – r can range from 0 to 1, with 0 meaning no relationship and 1 meaning a perfect relationship.

Now I don’t have a subscription to the journal where the full results are published, so I have not read them in detail. And I’m not denying that habits can be genetic, not at all. But here are some thoughts:

  1. The expectations of parents, teachers, and caregivers can make a huge contribution to a person’s development. Children work toward what is expected of them (if I had the time I would cite the many studies in education and human development that support this – I may add some laterhere’s one).
  2. Because my twins are identical, people often assume that they are the same in lots of ways – they expect them to behave in similar ways, and to develop at a similar pace. People will infer from fairly random or coincidental behaviours (like both lifting the same arm at the same time, or even the opposite arm at the same time), that they are behaving a certain way because they are identical twins. As I said above, children will work toward what is expected of them.

So with these two points in mind, it is no surprise to me that there is a stronger relationship in the study among identical twins than non-identical twins. Also let’s remember that parents completed the questionnaires about their children, so not only are children meeting parents’ expectations, but parents are self-reporting the extent to which their children meet their own expectations. I’d be interested to know if parents knew about the purpose of the study when they were completing the questionnaires.

Again, I’m not denying that habits can be genetic. I am skeptical however about the conclusion, based on this study, that 72% of picky eating is hereditary.

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