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"Breast-feeding mothers aren't going to go out and get into bar fights, but if someone is threatening them or their infant, our research suggests they may be more likely to defend themselves in an aggressive manner," study author Dr. Jennifer Hahn-Holbrook, a UCLA psychologist, said in a written statement.
What explains a phenomenon that some are calling the "mama bear" effect? The study suggests that the act of nursing blunts women's physiological reaction to perceived threats, tamping down fear and giving them extra courage to defend themselves and their babies.
For the study - published in the September issue of Psychological Science - researchers recruited three groups of women: 18 nursing mothers, 17 moms who used formula to feed their infants, and 20 non-mothers. As her baby was taken care of nearby, each woman was pitted against an overtly rude research assistant in a series of computerized time-reaction contests - and encouraged to celebrate each victory by pushing a button to deliver a sound blast.
What happened? The blasts delivered by the breast-feeding moms were twice as loud and as long as those given by non-mothers and nearly twice as loud as long as those given by bottle-feeding moms. The researchers viewed the bigger blasts as a sign of aggression.
And measurements of the moms' blood pressure showed that breast-feeders had pressures significantly lower than those of bottle-feeders or non-mothers. Lower blood pressure suggests reduced anxiety.
The so-called "lactation aggression" phenomenon has been identified in rats, mice, lions, deer, sheep, and other non-human mammals, Dr. Hahn-Holbrook said. But she said she believed hers was the first study to demonstrate the phenomenon in humans.
If breast-feeding helps moms protect newborns from danger, it's just one of many benefits of breast-feeding. In a call to action to support breast-feedingissued in January, the U.S. surgeon general said breast-feeding protects babies from diarrhea, pneumonia, and other infections as well as sudden infant death syndrome (SIDS) and subsequent obesity. In addition, moms who breast-feed are less likely to develop breast and ovarian cancer.
Mothers in numerous species exhibit heightened aggression in defense of their young. This shift typically coincides with the duration of lactation in nonhuman mammals, which suggests that human mothers may display similarly accentuated aggressiveness while breast feeding. Here we report the first behavioral evidence for heightened aggression in lactating humans. 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. Maternal aggression in other mammals is thought to be facilitated by the buffering effect of lactation on stress responses. Consistent with the animal literature, our results showed that while lactating women were aggressing, they exhibited lower systolic blood pressure than did formula-feeding or never-pregnant women while they were aggressing. Mediation analyses indicated that reduced arousal during lactation may disinhibit female aggression. Together, our results highlight the contributions of breast feeding to both protecting infants and buffering maternal stress.