Mammalian mothers who are nursing their young exhibit heightened aggression against threats to themselves or their babies. This reaction is termed ‘lactation aggression’, or sometimes ‘maternal defense’. Not surprisingly, considering that humans are mammals, women display a similar reaction.
Jennifer Hahn-Holbrook and her colleagues from UCLA gave a computerized time-reaction test to a group of volunteers, including 18 nursing mothers, 17 formula-using mothers, and 20 non-mothers. They each had to compete against an extremely rude rival, who was actually a confederate of the researchers. Each time one of the women won a round, she was allowed to punish her antagonist by delivering a sound blast. The nursing mothers blasted their rude competitors twice as loudly and for twice as long as the non-nursing women. In addition, the breastfeeding mothers’ blood pressure remained lower than the non-lactating women during the entire exercise. This correlates well with the observation that lactation suppresses the fear response in non-human mammals.
As Hahn Holbrook concludes:
[Breastfeeding] may be providing mothers with a buffer against the many stressors new moms face while at the same time, giving mothers an extra burst of courage if they need to defend themselves or their child.