Mom’s happiness [during a child’s infancy] was twice as important when it came to predicting mental health issues in boys. As for girls, only their mother’s happiness was found to be linked to their mental health as teens.
The article means “compared to Dad’s” but it’s not just compared to the father’s happiness. It’s compared to everything. Many mothers of small children don’t know that being selfish and seeking their own happiness is the best favor they can do to their kid.