Sometimes it is about the generalization power that different mechanisms have at different levels.

If one wants to explain why one particular member of a household developed anorexia rather than another particular member of a household then the explanation might look very different from when one asks:

Why is the prevalence of anorexia much much much higher in developed western nations than it is in developing nations?

Here one might attempt to appeal to genetic differences...

The trouble is that if you look at immigrants from developing to developed nations within two generations their prevalence looks the same.

What is left?

Why, the impact of skinny supermodels and cultural ideals, that is what. The appropriate explanation for it is going to be on the level of society.

Though in order to run a randomized double blind control trial on the effects of exposure to western ideals of beauty is something that can't be done for pragmatic / ethical reasons. And the pharma people go woo hoo! By making the randomized double blind control trial the 'epitome' of 'evidence based medicine' we have successfully biased things such that cultural mechanisms are effectively excluded from 'scientific' explanations! On with pharma ho!

You aren't going to find what the different cultures have genetically in common that raises the incidence of anorexia...
You probably aren't going to find what the different cultures have neurologically in common that raises the incidence of anorexia (the majority of the differences are caused by starvation rather than the cause of it)
But one doesn't need to reflect terribly much on differences in the ideal of beauty to see that we are getting SOME traction at least on an explanation.

Heritability of anorexia? Not sure that is a starter...
Unless, of course, you are looking at the heritability of a meme (cultural ideal).