MR. KOJO NNAMDIWe'll start this one with a question and ask you to respond by calling 800-433-8850. How do you think being a parent affects a man's masculinity, spending time with children, caring for children. 800-433-8850. How do you think the development of the nurturing side affects a man's masculinity? 800-433-8850. Men who see minivans and strollers as a threat to their masculinity may now have scientific proof.

MR. KOJO NNAMDIThe first large-scale study before fatherhood and after shows that men's testosterone levels drop significantly after they become parents, and the more involved men are in the care of their children, the steeper the drop. The studies' authors say this hormonal change could be nature's way of bringing out the nurturing side of men, and it may keep them more committed to sticking around and raising their offspring.

MR. KOJO NNAMDISome see is as proof that men too are biologically adapted to being parents. Joining us to discuss this is Christopher Kuzawa. He is a professor of anthropology at Northwestern University, and co-author of the study of men and fatherhood. He joins us telephone from somewhere Milwaukee and Chicago. Christopher Kuzawa, thank you for joining us.

DR. CHRISTOPHER KUZAWAGood afternoon.

NNAMDIAlso joining us is his co-author of the study, Lee Gettler, a doctoral candidate in anthropology at Northwestern. He joins us by phone from Chicago. Lee Gettler, thank you for joining us.

MR. LEE GETTLERThank you for having me.

NNAMDIChris, I'll start with you. I understand this is the first large-scale study to look at fathers and hormone levels. Tell us about it.

KUZAWAYeah. Well, in 2005 we measured hormone levels in about 800 men living in the Philippines as part of a longer-term study that's located in a city called Tabu there. And what we found is that fathers tended to have lower testosterone levels than non-fathers. Now, this is something that had been shown previously in other studies, and it's a fascinating finding, but it leaves certain questions open.

KUZAWAThere's several different ways that you could reach this result. One would be that becoming a father lowers your testosterone levels. Alternatively, the men who have low testosterone levels to begin with might be more likely to become fathers. And so because we are only looking at a snapshot in time, we couldn't really address those two possibilities. So what we did is about four-and-a-half, five years later we revisited these men and we measured their hormones again.

KUZAWAAnd at the beginning of the study they were about 22 years of age, so many of them were single, certainly were not fathers at that point, but in those five years, many had transitioned into fatherhood and had had offspring, and what we found, all of the men, if you look at the men as a whole, their testosterone levels all went down a little and that's what you expect with age.

NNAMDIBut the men who were new fathers saw a much greater decline in their testosterone levels, and the drop in their evening levels, the levels that you measure prior to bed, were about one-third lower. So this was the first study to show that becoming a father lowers your testosterone levels, and showed that it was fatherhood that lowered testosterone as opposed to low testosterone driving fatherhood.

NNAMDIAnd Lee, the effects were stronger, it would appear, the more time men spent caring for their kids. How did you measure that?

GETTLERWell, we had a series of care-giving behaviors that we asked the men about, and then the men kind of slotted them into these five levels of caregiving. So did they have any contact with their children, did they spend no time per day taking care of their kids, zero to one hour, one to three hours, or three or more hours. But the caregiving behaviors are really, you know, pretty straight forward and the kind of behaviors that you might expect, you know, a typical father in America to say that he does with his kids.

GETTLERThings like watching TV and playing games with his children, bringing them out on -- the children out on walks, bathing them and feeding them. So, you know, the fathers in this large metropolitan area in Tabu are really kind of doing typical fatherly things in the way that we think about them in the United States perhaps.

NNAMDIChris, there have been other studies measuring testosterone, how was this one different?

KUZAWAWas that question for me or for Lee?

NNAMDIFor Chris.

KUZAWAOh, okay. Yeah. I think what made this study different, number one, it's a larger study. Many of the studies that had been conducted in the past might look at 50 men or 100 men. We followed 800 men, so it's a larger study, and again, I think the thing that really makes the study unique is that we followed men through time. We measured their hormone levels and the change in the hormone levels as they became fathers, and so that's what really allows us to say what is cart, what is horse in this relationship. So I think it's very exciting.

NNAMDIChris, I'll stay with you for a second. A man's behavior actually affects his hormone levels. How significant is that?

KUZAWAHow a man's behavior affects his hormone levels. Well, from our study, and based upon, you know, what Lee was just talking about, we've got evidence that the more time that you spend with your children the lower the hormone levels. Now, beyond that, you know, I think there's a lot of speculation. We can only speculate as to what might be driving that.

KUZAWABut interacting with the child, the kinds of keys that you experience when interacting with the child and bonding with the child, these are all factors that might psychologically lead to body to lower testosterone levels, but we -- we'll need to follow this us in -- with the study moving forward.

NNAMDII'd be fascinated to hear how our listeners respond to the findings of this study. You can call us at 800-433-8850. A study showing that the testosterone levels of men decline on the basis of time spent caring for and spent with their children. 800-433-8850. Lee, so far what kind of reaction have you gotten to this study?

GETTLERI feel like in general the reaction has been overwhelmingly pretty positive. You know, I think we have this kind of tendency to think about mothers as being innately nurturing or biologically oriented to be parents, and fathers in our culture are sometimes portrayed as being inept and not built for parenthood.

GETTLERBut, you know, I think our study kind of adds a really interesting caveat to that simple model and shows that men actually have this kind of built-in biological ability to respond to child care, and that men are also, to a certain extent, and in their own way, designed to be involved with child care to the extent that when men kind of make this active choice to help mothers take care of their children and be involved in a hands-on way, their biology actually gets on board with that and responds in a way that we think might, you know, perhaps increase men's sensitivity to their children's needs, or kind of attune them to children in these really early years when they're, you know, in infancy or in their toddler stages when they're just incredibly dependent on their caregivers.

NNAMDILee Gettler is a doctoral candidate in anthropology at Northwestern University, co-author of a study of men -- on men and fatherhood. He joins us along with his co-author Christopher Kuzawa, a professor of anthropology at -- Kuzawa -- a professor of anthropology at Northwestern University. Their findings indicating that the testosterone levels of men drop with their involvement in spending time and caring for their children. We go to Dianna in Montgomery County, Md. Dianna, you're on the air. Go ahead, please.

DIANNAHi, Kojo, thank you. First of all, I think it's a very interesting study. I'd be interested to hear what the ages of the children involved here, whether they were teenagers or infants. Second I find it odd that I guess during the opening I took from your comments, Kojo, that you were saying masculinity and levels of testosterone are equivalent. I find that very odd.

DIANNAMy own husband helped raise our three wonderful children with everything from holding them, feeding them, to changing them, and everything else that went along the way, and, he, you know, yesterday afternoon he was out with a chainsaw, and the day before that he was fixing the ceiling in our kitchen. So, I'm not sure I get the correlations that was implied in the introduction.

NNAMDII guess I was talking about traditional ideas about masculinity, because having been a single parent myself, I did a lot of the things that your husband is currently doing. Here -- Dianna, thank you so much for your call, and I would like to hear how Christopher Kuzawa responds to that.

KUZAWAYeah. Well, I'd like to answer -- these are really wonderful questions. With respect to the issue about masculinity, I think it's important to emphasize that the decline in testosterone that we see in fathers, although it's substantial, it still is absolutely within the normal range. There's not gonna be an effect on for instance something like sperm production or libido.

KUZAWASo this is not really an impingement to masculinity. It's more about kind of tuning, as Lee mentioned a second ago, kind of tuning one's attention for instance and making a male more responsive to the kinds of cues that babies and infants and children provide to us about their needs. There is some evidence that, you know, lower testosterone levels do make men more responsive.

KUZAWASo I think that there are certain benefits from having lower testosterone, but it shouldn't be seen as something that's going to impinge upon a man's masculinity, or the kinds of things that men may value. With respect to the age of the offspring, actually Lee is going to have probably a little bit more to say about this, but in general these are fairly very young, most of the fathers were, you know, as I said in the beginning, about 22 years of age, by the end they were about 27, and so the effect of the age of the child, the youngest child we see in general that the testosterone levels are lower in fathers and attempts to be fairly stable for the first couple years.

KUZAWAWe did not have a lot of fathers with quite a bit older children, and so that's something that we could follow up with. In the future if we can get another testosterone measurement as these men's -- as their children age further, and especially those men who don't have new children, I think it will be happen to see what happens with the testosterone levels. But at this point, we really don't know how stable this is over time.

NNAMDIOkay. Dianna, thank you very for your call. Lee, hold your thought for a second, because I would like you to respond to Nat in Blacksburg, Va. Nat, you're on the air. Go ahead, please.

NATHello, how you doing. I just -- my main issue is that I understand how testosterone affects a man, and it's something that is a result of him being a man, but I've been taking care of my children for years working with them on site with me and, for example right now, I'm cleaning up a tree that's come down from Hurricane Irene. I -- and my daughter's with me of all, you know, it's not my two sons, it's my actual daughter, and she is okay. Everything's fine, I'm taking care of her, and whether or not you have that testosterone kick in, I don't see why that would even matter as it comes to you being a man, and it's not really me making a statement.

NNAMDIOkay.

NATI'm just concerned that...

NNAMDII -- we're running out of time, so allow me to have Lee Gettler respond to you. Lee, what would you say to Nat. Why does this even matter?

GETTLERWell, I think the idea, and I hate to put any words in Nat's mouth, by any means, but this is sort of where I thought he was going, is simply that, you know, this comes back almost to the previous comment about testosterone and masculinity. I think that's kind of a popular conception in the United States that testosterone equates to manlihood or masculinity, but, you know, it's really much more complicated than that.

GETTLERI mean, I think in some ways if you take another hormone, estrogen, that's thought of as being a female hormone, but in fact, in men, a lot of the effects of testosterone are actually through estrogen. So testosterone becomes estrogen and has a lot of effect on men. So I think some of our popular conceptions about these hormones are a little bit misleading, and Chris was speaking to this.

GETTLERI don't think that because a father might have a decline in testosterone, that that in any way sort of impinges on who he is as a man or what he can do as a father. I think Nat's kind of examples of him working his children, I mean, it's entirely plausible if Matt was a very involved father that maybe Matt experienced some of these declines of testosterone. But, you know, it was -- he has no idea, it was unconscious.

GETTLERIt's not as if he's aware of it, but it may have made him slightly more sensitive to his children's needs, and it didn't, you know, impinge on his ability to be working with his hands and being kind of an active masculine man as he had been before he had children. And again, I don't mean to put words in Nat's mouth, but that's kind of what I interpreted him to be saying.

NNAMDIThank you very much for your call, Nat. Chris, it's my understanding other studies confirm this causation since apparently divorce or separation leads to an increase in testosterone. What are some of the theories as to why this might be? We only have about a minute left.

KUZAWAWell, our findings I think suggest that because this is part of our biology, it's had enough time to actually become part of our biological makeup evolutionarily. And if you look at other species in which males are involved in raising their children, especially in the case of birds, there's a lot of birds in which this is the case, you see the testosterone levels generally decline around the time that they start taking care of offspring.

KUZAWAIt's high during mating season, and then it's lower during periods when they're caring for young. And so the pattern that we see in humans is very similar to what we see in these other species, and I think the general idea is that testosterone kind of orients you away from the child and in a way it orients you more towards for instance finding a mate, opportunities out in the social realm, and perhaps distracting you from the kinds of requirements that are needed of your child.

NNAMDIAnd I'm afraid that's -- that's about all the time we have.

KUZAWAOkay.

NNAMDIChristopher Kuzawa is a professor of anthropology at Northwestern University. He co-authored to study on men and fatherhood with Lee Gettler, a doctoral candidate in anthropology at Northwestern University. Gentlemen, thank you both for joining us.

NNAMDI"The Kojo Nnamdi Show" is produced by Brendan Sweeney and Michael Martinez, Ingalisa Schrobsdorff and Tayla Burney with assistance from Kathy Goldgeier and Elizabeth Weinstein. The managing producer is Diane Vogel. Our engineer, Andrew Chadwick. A.C. Valdez has been on the phones. Thank you all for listening. I'm Kojo Nnamdi.