Honestly, it could be any number of things - they literally can't take care of the child, whether due to poverty, mental health, drug or alcohol use, the mother might not want the father there for a number of reasons, in a mother leaving the child's case, the child might be a result of rape... But it IS hard on the parent. I don't know of any studies out there, but in the experience of the person who was at my class today who is in charge of COUPLES-based parenting and involving the father where he may have been absent for any number of reasons before and trying to help eliminate those reasons, those fathers become a lot happier when they are actually involved with their children. It's like they get something back in their lives that they were meaning. It seems to be especially the case for men with multiple children by multiple women.