Testing skin color bias with dolls

An anthropology blog writes:

Light- and dark-colored dolls have long been used to study how children acquire negative attitudes to dark skin. ... A doll test typically involves asking a child to choose between a lighter-colored doll and a darker-colored one. ... The first doll studies are well known today, even among non-academics.

You might think that these experiments show how kids learn racial prejudice from their parents. But there have been a lot of studies, and they do not show that at all. The preferences depend on age and sex, but are similar throughout the world regardless of what the kids are taught.