Is Fatherhood Bad for Men?

Research shows men's testosterone levels drop after they have kids.
3:00 | 12/20/11

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Transcript for Is Fatherhood Bad for Men?
Close what the biggest changes in family life in the past hundred years fathers getting involved with raising the kids breakfast table -- bath time. Dad is frequently in charge. It may seem like a great piece of news to some mothers but new research suggests there might be -- hidden costs for the men. -- on the -- the core of their identity here's ABC's nick watt in London on the frontlines. Of fatherhood. It's. That's me and my little -- his name is mind. Certainly it is Superman -- big twelve hours a day. Give him cake and. I listen to news stories yeah. Yeah yeah yeah -- and and. -- -- -- -- -- -- -- -- -- -- -- Well -- it's not here. And that's it anyway -- liked incumbent pretty damn good breaking. But scientists have discovered something that's -- Oldest modern man daddy is no or it might -- -- and dramatic. -- so this is making me. Acts of them. I think it's absolutely the wrong reaction -- I've been. Hearing that a great -- from a lot of people. We -- -- testosterone as manly and eating sperm production and libido create new lean muscular bodies and -- aggressive personalities. Testosterone helps a -- them -- a mate and offspring. But those same things can get in the way of being a good father like many species of -- human males have evolved to have to. -- -- -- -- -- -- And father. If a man couldn't modulate his testosterone in his temper might be shorter might be very bad babies -- -- researchers from Northwestern University studied men in the Philippines and conclusively proved testosterone. Nose -- went to at least take care of kids. Fathers have on average 20%. Less testosterone and the norm fathers that right now -- -- the weekend and hang out with him all the time. My testosterone in his rock rock -- but what's really clever about it. It's only ten -- And here's the group. Its London office -- He left an illustrious he'd been a game they need for it. Howard and a great faith. Stops to him. Meant -- that's broad and -- number food he got him growth. Is about and next. People. And quite -- and -- And -- are you David dirty -- plan. People might say about it -- disagreement possibly the truth. During the day when he's with its kids -- testosterone and he -- when he's on the field. -- but why aren't dads so be it pro coaches decade was born just two weeks ago. Have you noticed any changes in yourself since she was absurd infested with human parenting is easier as -- to Paris and to all our kids are so helpless. And that's why men and have evolved to drop the -- -- -- -- And mock him. To him in the 1970s. Nothing my own dad never made me and she never changed a diaper was sent to boarding school -- sixty years old. In the 1970s men old today bits instead of -- but it kids -- we -- them back. -- not always an alpha male always with high testosterone always out there. Seeking mates and so forth that that's a very chimpanzee like version of humans and health benefits low testosterone might cut prostate cancer rates -- nose looked at -- we haven't talked to look at. Studies like this that suggest. That we should look at it. Bottom line this is in fact more manly -- leaving wife and kids at home to skydiving. Skirt chasing on the way chicken and -- and -- lucky ducky lucky it's -- he met. C. CDC. During the first few months of a baby's life so they say -- that's testosterone is at its lowest. And my wife. -- -- -- They beat Sebastien. My testosterone right now and is in the face. Arising from me and -- in the next nature's. Actions but I'm -- looked for Nightline in.

This transcript has been automatically generated and may not be 100% accurate.

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