To clarify on this point, since your objection is correct: I don't think the scientist is biased without a proper thought, but the article that led me to the meta-analyse implied the ''gender differences exist'' stream of thought was biased by correct gender roles, so I asked myself if bias from the other was also a possibility once this kind of logic is deployed.
Nurture in adults is indeed a difficult question, although you can always observe children.
However, there's is a famous case of david reimer, better know as john/joan, who was raised as a girl after his penis was burned after a failed circumcision. He/she ended up hating being a female, in spite of being raised a such.
It's a interesting topic, but it remains difficult because it has a lot to do with how society is structured.