One interesting vein of research was presented at the At-Home Dads Convention a couple of weeks ago in Chi-town. Rebel Dad mentioned it, and now, Peter Baylies, the at-est-home dad of them all, expands on it.
Turns out that pregnancy, birth, and the close proximity of a newborn child are enough to trigger changes in hormone levels and brain activity in men. The findings were presented by Dr. Kyle Pruett, a longtime fatherhood and child development researcher as the conference keynote speech.
Check out Rebel Dad's original conference post, Baylies's account of the speech, the Psychology Today article that covers some of them, and Pruett's book, Fatherneed, which predates them.
but it changes in a good way to strengthen the bond between mom, baby and daddy, so the baby is kept safe and well fed (and proves men CAN and maybe should breastfeed).