Maternal Defense Breast Feeding Increases Aggression by Reducing Stress

Jennifer Hahn-Holbrook; Julianne Holt-Lunstad; Colin Holbrook; Sarah M. Coyne; E. Thomas Lawson

doi:10.1177/0956797611420729

Maternal Defense

Breast Feeding Increases Aggression by Reducing Stress

¹Department of Psychology, University of California, Los Angeles
²Psychology Department, Brigham Young University
³Center for Behavior, Evolution, and Culture, Department of Anthropology, University of California, Los Angeles
⁴School of Family Life, Brigham Young University
⁵Institute of Cognition and Culture, Queen’s University Belfast

Jennifer Hahn-Holbrook, University of California, Los Angeles, Department of Psychology, 1285 Franz Hall, Box 951563, Los Angeles, CA 90095 E-mail: jhahn{at}psych.ucla.edu

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.

Article Notes

The authors declared that they had no conflicts of interest with respect to their authorship or the publication of this article.
Financial support from the Institute of Cognition and Culture at Queen’s University Belfast, the Women’s Research Institute at Brigham Young University, and the U.S. Air Force, Office of Scientific Research (Grant FA8655-09-1-3065) is gratefully acknowledged. The first author was supported by the MH15750 training fellowship in Biobehavioral Issues in Mental and Physical Health at the University of California, Los Angeles, during work on this project.