Lead author Jennifer Hahn-Holbrook, a postdoctoral fellow at UCLA says when breastfeeding women behave aggressively, they register a lower blood pressure than other women.
The study, published in the journal Psychological Science, suggests breastfeeding can help dampen the body's typical stress response to fear, giving women the extra courage they need to defend themselves.
"Breastfeeding has many benefits for a baby's health and immunity, but it seems to also have a little-known benefit for the mother," Hahn-Holbrook says in a statement. "It 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."
However, the aggression demonstrated -- dubbed the "mama bear" effect -- by breastfeeding mothers has its limits, Hahn-Holbrook says.
"Breastfeeding 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," Hahn-Holbrook says.
Researchers recruited three groups of women -- 18 nursing mothers, 17 women who were feeding formula to their babies and 20 non-mothers. Each was asked to compete in a series of computerized time-reaction tasks against a research assistant posing as an overtly rude study participant, while the infants were supervised in an adjoining room.