It's not clear from the summary or a very quick glance at the articles whether the researchers controlled for sex differences or impact on parent attitudes towa

It's not clear from the summary or a very quick glance at the articles whether the researchers controlled for sex differences or impact on parent attitudes towards fraternal twins. It is sensible to assume that one sex will imitate dad while the other will imitate mom, or that one toddler receives more attention than another from either parent. Might anyone familiar with the topic have some insights on this?