Entering the parenting field, I read a lot of studies which only involved women. It seemed reasonable: if you have a child, there's almost always a female parent that can be asked to include in the study, and women receive/need maternity leave so can more easily participate in studies especially those that start in the hospital itself.

While reading the studies, I had to assume that they were applicable to men too but I didn't know for sure.

More recently, in the EU, they've been replicating those same studies using fathers (e.g. parental attachment to newborns following skin-to-skin), and finding out that yes the results hold true for men.

It's important to include both genders, but lacking that... we can assume the same effects are true.