


But what about humans? A newly published study has revealed the first behavioral evidence for heightened aggression in nursing women. What's more, these women are not only more likely to display aggressive behavior than their formula-feeding peers, they actually exhibit lower systolic blood pressure in the process. Translation? Breast feeding can help you defend your young and help keep you de-stressed.

The researchers' hypothesis — that human mothers would display accentuated aggressiveness while breast feeding — was based on prior research in non-human mammals that shows that lactation enables heightened defensive aggression by decreasing the animal's fear response. Evolutionarily speaking, the ability to react defensively is enormously beneficial to the fitness of a mother and her offspring.


But the scientists couldn't exactly pit these women against legitimately harmful threats, so how did the researchers screen for aggression? Simple: a videogame tournament.

Well, not videogames, per se. Eighteen nursing mothers, 17 formula-feeding moms, and 20 non-mothers were recruited to participate in a computerized, competitive, reaction-time test (okay, a videogame) against a research assistant posing as an overtly confrontational study participant (try to imagine an unnecessarily hostile, pre-teen XBox Live opponent on Call of Duty).


Here's where things get awesome. If the study participant won a round of the competition, she was allowed to press a button that delivered what the researchers describe as a "punitive sound burst" to their overtly hostile opponent (the trained research assistant). For those of you wondering, the "punitive sound burst" in this instance is roughly the equivalent of "booyah, bitch," and has, in fact, been validated as a measure of physical aggression.

The paper's authors describe their findings:

Breast-feeding mothers inflicted louder and longer punitive sound bursts on unduly aggressive confederates than did formula-feeding mothers or women who had never been pregnant.

In fact, mothers who exclusively breast-fed their infants were found to be almost twice as aggressive as formula-feeding women and non-mothers. (It's worth noting that formula-feeding women did not exhibit more aggressive behavior than non-mothers.)

The researchers believe the tendency for breast-feeding mothers to dish out longer and louder punitive sound bursts is mediated by a lactation-related decrease in the body's response to stress. The researchers explain:

Exclusively breast-feeding mothers had lower [blood pressure] during the aggressive encounters relative to the other groups, and [blood pressure] correlated inversely with aggressive behavior. Together, these findings suggest that in humans, as in many other mammalian species, lactating mothers are more likely to aggress against hostile conspecifics than are non-lactating mothers or [women who have never given birth], at least in part because they experience dampened arousal in response to stressful aggressive encounters.

So what's the take home message?

"Breast-feeding mothers aren't going to go out and get into bar fights," said UCLA's Dr. Jennifer Hahn-Holbrook, lead author of the study. "But if someone is threatening them or their infant, our research suggests they may be more likely to defend themselves in an aggressive manner."

The researchers' paper, "Maternal Defense: Breast Feeding Increases Aggression by Reducing Stress" appears in the September issue of Psychological Science

Images via Jason Stitt/Shutterstock

Thanks to Dr. Hahn-Holbrook for the forwarded copy of the paper