An observation that has been sitting on my back burner for awhile is the apparent phenomenon that mothers tend to be more responsive to their babies' cries than fathers. I don't mean this in the sense of a behavioral response, but rather, as a physiological response. For example, I'm amazed at how loud our daughter has to cry at night to wake my husband up - sometimes I hear it through ear plugs before he seems to notice.

Of course, this is a single observation. Ever the scientist, I formed two possible hypotheses about this phenomenon, which, based on conversations with other couples with small children, seems to be fairly widespread. One hypothesis is that it is usually the mother who has the heightened response, and that it could be a hormonal issue. (It probably is partly associated with breast feeding, but I stopped nearly a year ago and still have the response.) Another hypothesis is that it is the primary caretaker who has the heightened response, and just seems like moms because, surprise, they are usually the primary caretakers.

My bit of unscientifically collected data supporting the first hypothesis rather than the second is that out of probably half a dozen couples, it is the mothers who have the response, even though in two of these cases the father has been the primary caretaker. So I'm guessing there is some real physiological basis in this.

Real scientific studies could tell me whether I'm right or not. Surprisingly, though, an hour plus of searches both on Web of Science and Google produced nothing concrete on this topic. The studies I did find were nearly always on mothers only (which I found annoying), and mostly compared mothers and non-mothers, or especially, mothers of "collicky" vs non-collicky babies. A few studies, below, were a bit more interesting to me.

First:

Purhonen M, Kilpelainen-Lees R, Paakkonen A, Ypparila H, Lehtonen J, Karhu J, 2001. Effects of maternity on auditory event-related potentials to human sound.
NEUROREPORT 12 (13): 2975-2979 SEP 17 2001

Abstract: Auditory event-related brain potentials (ERPs) were recorded in response to an emotional (a baby's cry) and a neutral (a word) stimulus in a group of mothers 2-5 days after childbirth (n = 20) and in control women (n = 18) who were not in the state of early motherhood. For each mother, her own infant's cry was recorded and used as the cry stimulus, whereas a strange baby's cry was used for control women. The word stimulus was identical for both groups. Stimuli were presented in intermittent trains in order to study the arousal responses to the first stimuli of the trains, and refractoriness of ERPs during stimulus repetition. The N100 responses were significantly larger in amplitude in mothers than in control women, not only to the emotional cry stimuli but also to the neutral word stimuli. The finding suggests a general increase in alertness and arousal in mothers, which may be necessary in enabling the mother to be continuously alert to her infant's needs. This allows good care of the infant and may be essential in building an emotional tie between the mother and her child.

I think this study also needed an additional test group of mothers responding to cries of strange children. The conclusion (about the "emotional tie between the mother and her child") may not quite be supported if any baby's cry elicits the same response. Or perhaps it is, given that mothers are going to be exposed to their own baby's cries more often. Because we are a social species, though, that would be relevant and interesting information.

It's not surprising mothers and non-mothers differ on this of course, but what about fathers? I really think this is an obvious thing to test. Aren't "emotional ties" with dad important? (I checked these authors and there was not a paternal follow-up to this paper.)

Here's another dealing with mothers' brains - it appears to be an unpublished pilot study that I found online. It suggests that the cingulate region of the brain (part of the limbic system, which is associated with emotional respons) is involved with maternal response to cries. Wish they had done it with fathers too!

Feasibility Of Using fMRI To Study Mothers Responding To Infant Cries.
J.P. Lorberbaum, J.D. Newman, J.R. Dubno, A.R. Horwitz, Z. Nahas, C. Teneback, M.R. Johnson, R.B. Lydiard, J.C. Ballenger, M.S. George.

This small pilot experiment demonstrates the feasibility of studying maternal response to infant cries in a fMRI scanner environment. It tentatively supports the notion that the cingulate is involved in response to infant crying. Consistent with some monkey lesion work, it suggests that the ROF may also be involved (2). In designing follow-up studies, we are considering such factors as maternal and infant temperament, the type of infant cry, using each mother's infant's own cries, the appropriateness of our control stimulus, postpartum timing, and allowing a mother to actively terminate the cries as in natural settings. Future work in this area may lead to understanding the brain basis of mother-infant interaction and the biological roots of child neglect and abuse.

Here's one that looks at fathers vs. nonfathers:

Fleming AS, Corter C, Stallings J, Steiner M, 2002. Testosterone and prolactin are associated with emotional responses to infant cries in new fathers.
HORMONES AND BEHAVIOR 42 (4): 399-413 DEC 2002

Abstract: To determine the responsiveness of new fathers and non-fathers toward infant cues, we exposed fathers and non-fathers to infant cries and to control stimuli and we measured affective, heart-rate, and endocrine responses, including salivary testosterone and cortisol and plasma prolactin concentrations prior to and after cry presentations. We found that (1) fathers hearing the cry stimuli felt more sympathetic and more alert compared to groups who did not hear the cries or to non-fathers who heard the cries; (2) fathers and non-fathers with lower testosterone levels had higher sympathy and/or need to respond to the infant cries than fathers with higher testosterone levels; (3) fathers with higher, as opposed to lower, prolactin levels were also more alert and more positive in response to the cries; (4) fathers hearing the cry stimuli showed greater percentage increase in testosterone than fathers not hearing the cry stimuli; (5) experienced fathers hearing the cries showed a greater percentage increase in prolactin levels compared to first-time fathers or to any group of fathers hearing control stimuli; finally, (6) partial correlations with parity and experience entered as a covariates indicated that both experience and testosterone contributed to the variance in fathers' affective responses to infant cries. Taken together, these results indicate that, as with a number of other biparental species, human fathers are more responsive to infant cues than are non-fathers and fathers' responses to infant cues are related to both hormones and to caregiving experience.

While the previous papers looked at brain response, this one studied hormones, which makes it interesting, as it is not necessarily intuitive that fatherhood affects hormones. But, not only do having less testosterone and more prolactin make a father more responsive, but the cries themselves affected hormone levels, indicating a true physiological response to the sound of a baby crying. While one might expect maternal hormones to do this, it is more interesting to find it in fathers too.

From looking at this paper and the following...:

Storey, A. E., Walsh, C. J., Quinton, R. L., and Wynne-Edwards, K.E.(2000). Hormonal correlates of paternal responsiveness in new and expectant fathers. Evol. Hum. Behav. 21,79-95.

...it becomes obvious that the situation is not clear-cut; men who naturally have more testosterone are less responsive to their babies crying. It is not too hard to imagine couples out there where the father is the one leaping out of bed at the slightest peep.

Finally, I found a report (pdf) available here, which aims partly to look at brain responses to baby cries of both men and women, with and without children - exactly what I was looking for. Sadly, it is only a progress report from 2001, and my searches on the authors names came up empty on any follow-up. But again, the cingulate gyrus is implicated.

Here are the questions I would like to see addressed:

What effects do breast feeding or not have on physiological response to baby cries? Looking at hormones would be confounded by the hormones associated with the breast feeding, but an fMRI might be interesting.

What is the effect, if any, of actually having given birth to the baby, vs. adoption? (I suspect there has probably been some research on this aspect, but I haven't taken the time to delve into it.)

And of course, I would like to see the fMRI data for the test groups mentioned in the progress report above.

To close, here is my list of all the things that often sound, to me, like a baby crying (and my husband thinks I'm nuts, of course):

Dogs barking, cats meowing, trucks and cars going by on the freeway, train whistles, sirens, wind, kids playing outside, various large appliances, lawn mowers, snow blowers, leaf blowers, and the flywheel on my rowing machine.

That's all I could think of off the top of my head. I will edit the list as I think of more things. Please comment if you can add to the list or have any data to back up or refute my groundless assertions!

Labels: brain, gender, humans