Breast-feeding mothers are far more likely to demonstrate a 'mama bear' effect', aggressively protecting their infants and themselves , than women who bottle-feed their babies or who are non-mothers, says a new study in Psychological Science.

To help mama bear out, breast-feeding mothers register lower blood pressure than other women when behaving aggressively - the researchers say suggests breast-feeding helps dampen the body’s stress response to fear; breast-feeding mothers are more likely to be courageous.

The breast-feeding mothers’ reaction is known as “lactation aggression” or “maternal defense” in mammals.  Non-human female mammals, including macaques,  mice, sheep and others, display more aggression when they are lactating than at any other reproductive stage, so they set out to test that reaction in people. 

For the study, researchers recruited three groups of women — 18 nursing mothers, 17 women who were feeding formula to their babies and 20 non-mothers. Each woman was asked to compete in a series of computerized time-reaction tasks against a research assistant posing as an overtly rude study participant. The womens' infants were supervised in an adjoining room.

Upon winning a round in the competition, the victor was allowed to press a button and deliver a loud and lengthy “sound blast” to the loser — an act of aggressiveness. The researchers found that breast-feeding mothers delivered sound blasts to the rude research assistant that were more than twice as loud and long as those administered by non-mothers and nearly twice as loud and long as those by bottle-feeding mothers. This was true both before and after the breast-feeding mothers nursed their infants.

The researchers also measured participants’ stress levels via blood pressure during the experiment. Breast-feeding mothers’ systolic blood pressure was found to be approximately 10 points lower than women who were feeding formula to their infants and 12 points lower than non-mothers.

Previous research in non-human mammals has shown that lactation enables heightened defensive aggression by down-regulating the body’s response to fear, a phenomenon that benefits the survival of both mothers and their offspring. The lower blood pressure seen in the breast-feeding mothers during acts of aggression, the researchers say, is an indication that the same mechanism is likely at work in humans as well.

“Breast-feeding has many benefits for a baby’s health and immunity, but it seems to also have a little-known benefit for the mother,” said Jennifer Hahn-Holbrook, a postdoctoral fellow in the UCLA Department of Psychology and the study’s lead author. “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.”

Breast-feeding mothers has its limits, Hahn-Holbrook added. “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.” 

Co-authors of the study included Julianne Holt-Lunstad, a psychology professor at Brigham Young University; Colin Holbrook, a postdoctoral fellow and research associate in the UCLA Department of Anthropology; Sarah Coyne, a professor of family life at Brigham Young University’s School of Family Life; and Ernest Lawson, a professor at Queen’s University Belfast.