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Notably, they didn't control for social effects by looking at female and male sibling pairs with an age difference of 10-18 months. How much of this is parents favoring the male sibling?

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"To separate the effects of fetal testosterone from postnatal socialization, the research team repeated their analyses focusing only on female twins whose twin sibling—either twin sister or twin brother—died shortly after birth, and thus they were raised as singletons."

This was my immediate concern as well. But they addressed it.

Shouldn't they have monitored both groups, using twins raised together as the control group?

"repeated their analysis"

This effect exists in twins raised as singletons and twins raised with their brother.

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