Lactating Moms More Aggressive, Explains Tim Loving

Human ecology professor Tim Loving blogs about maternal aggression.

If there’s one thing I learned from the old Mutual of Omaha’s Wild Kingdom, it’s that mothers don’t look too kindly on people messing around with their babies.

Across many non-human species, lactating moms’ maternal aggression has been well-documented. The general thinking is that because most young mammals are particularly vulnerable during the developmental period that coincides with lactation, nursing moms are capable of uncharacteristic levels of aggression if that’s what it takes to protect their youngins’. Interestingly, there’s been very little research to determine whether human moms are more likely to go all “Wild Kingdom” when lactating. Until now.

In a soon to be published study (1) in the journal Psychological Science, Jennifer Hahn-Holbrook and colleagues creatively tested the effect lactation has on moms’ aggressive behavior towards others. Specifically, they put (a) exclusively breastfeeding moms (those who had been breastfeeding exclusively for up to 6 months), (b) ‘mixed-feeding’ moms (those who were breastfeeding and using formula), (c) non-lactating formula-feeding moms, and (d) childfree women (obviously, non-lactating) in a situation that would provoke the ire of even the most mild-mannered Mrs. Cleaver.

Here’s the setup: the women (i.e., participants) arrived to the lab to find another ‘participant’ there as well. The other participant, actually a confederate, was in cahoots with the researchers. To increase the likelihood that the real participants would have some incentive to act aggressively towards their competition (more on this below), the confederates were trained to ignore the participants, obnoxiously chew their gum, and play on their cell phones while the researcher explained the upcoming task. The participants were then led to believe they would be competing with the confederate in a ‘dot-probe task’, during which they raced the confederate to indicate where on a computer screen a dot appeared. What did they win? Since it isn’t ethical to allow women to punch a fellow participant, they won the opportunity to punish the loser by administering a loud ‘sound burst’ to the loser. After each round, the winner chose both the volume of the sound (up to 105dB – this is roughly equivalent to the loudness of a jackhammer) and how long it lasted (up to five seconds). Do not pass go. Do not collect $200.

In reality, the participants were actually playing a computer rather than the confederate; the key is that participants believed they were competing against the rude, obnoxious woman they met at the start of the study. All women, regardless of their lactation status, won and lost the same amount of times. Researchers measured how much the women punished the confederate (via length and intensity of the noise) and the participants’ blood pressure before, during, and after the task. Importantly, the women competed in the game twice: once before feeding their infants, and once afterwards, which allowed the researchers to determine whether it is the act of breastfeeding vs. lactating more generally that affects aggression.

The exclusively breastfeeding mothers were nearly twice as aggressive compared to formula feeders and childfree women, who did not differ in aggressiveness (i.e., it is lactation, not motherhood, that increases aggressive capabilities). Importantly, the act of breastfeeding did not increase the levels of aggression for lactating moms; they were equally aggressive before and after breastfeeding. This makes sense: mom can’t always ask an intruder or other threat to wait a few minutes so she can breastfeed her child before fighting back.

What is the cause of lactating moms’ aggression? The authors speculated that lactation helps women confront stress more adaptively, thus facilitating their aggression. Think of it this way: when we are threatened, we typically experience fear, and that fear encourages us to engage in ‘flight’ (or avoidance) rather than ‘fight'. But, if we experience less stress when we encounter a threat, it’s easier to engage the threat rather than run away from it. Indeed, exclusively breastfeeding moms had lower blood pressure when they were punishing the confederate, and that lower blood pressure facilitated moms adopting a ‘fight’ rather than ‘flight’ response.

--

(1) Hahn-Holbrook, J., Holt-Lunstad, J., Holbrook, C., Coyne, S. M., & Lawson, E. T. (in press). Maternal defense: Breast feeding increases aggression by reducing stress. Psychological Science. Published online 26 August 2011 DOI: 10.1177/0956797611420729.