Maternal defense: breast feeding increases aggression by reducing stress

Abstract

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.

Conflict of interest statement

Declaration of Conflicting Interests

The authors declared that they had no conflicts of interest with respect to their authorship or the publication of this article.

Figures

Fig. 1

Mean aggression before and after the feeding period in the three groups of women (exclusively breast feeding, formula feeding, and nulliparous; the latter group took a break during the feeding period). Error bars represent standard errors of the mean.

Fig. 2

Mean (a) systolic blood pressure (SBP) and (b) diastolic blood pressure (DBP) during the two aggressive encounters in the three groups of women (exclusively breast feeding, formula feeding, and nulliparous). Error bars represent standard errors of the mean.

Fig. 3

Scatter plots (with best-fitting regression lines) showing the association between aggression and mean (a) systolic blood pressure (SBP) and (b) diastolic blood pressure (DBP) during the aggression paradigm.