I have explained how you can estimate the effects of genes on traits that vary with data from adopted kids or with data from MZ and DZ twins. Both of these study designs have limitations, which, depending on your point of view, either have little impact on the results, or render the results meaningless.
I’ll briefly mention several potential problems with each study design before introducing a third study design hat potentially avoids all of these problems.
Potential problems with adoption studies
- Data from the biological parents of adopted children are hard to come by. An adoption study is still doable provided one has data from at least one adoptive parent as well as from the adopted kid herself. The researcher will have to assume that the assignment of children to parents occurred essentially at random. This assumption is sometimes tenable, sometimes not.
- Adoptive parents are sometimes biologically related to their adopted children in some way (e.g. an aunt or uncle adopts his/her niece when the niece’s parent dies or becomes unable to care for the child.) In this case, adopted kids may behave more similarly to their adoptive parents simply because they share some of their genetic make-up.
- Adoptive parents may treat their adopted kids differently than they treat their biological kids. Evolutionary psychology would predict this, although there’s evidence against it (http://asanet.ba0.biz/images/press/docs/pdf/Feb07ASRAdoption.pdf). If parents invest less in their adopted kids than they invest in their biological kids, then we might expect any effect of parental socialization on adopted kids to be muted to some degree.
Potential problems with the classic twin study
Since MZ twins are genetic clones, they tend to:
- Experience very similar treatment from everyone who stands to influence them
- Spend a great deal of time together
- Experience a powerful bond, perhaps unlike the bonds experienced by any other pair of human beings
Meanwhile, DZ twins share no more genes than non-twin full siblings, so they will tend to experience relatively less similar treatment, spend less time together, and experience less powerful bonds than MZ’s.