ScienceDaily (Aug. 31, 2011) - Women who breastfeed are far more likely to
demonstrate a "mama bear" effect - aggressively protecting their infants and
themselves - than women who bottle-feed their babies or non-mothers, according to a new
study in the September issue of Psychological Science, a journal of the Association for
And when breastfeeding women behave aggressively, they register a lower blood
pressure than other women, the study found. The results, the researchers say, suggest
that 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," 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."
But the aggression demonstrated by breastfeeding mothers has its limits,
"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," she said.
The breastfeeding mothers' reaction is known as "lactation aggression" or
"maternal defense" in mammals.
Hahn-Holbrook was aware that non-human female mammals, including macaques, rats,
mice, hamsters, lions, deer, sheep and others, display more aggression when they are
lactating than at any other reproductive stage, but she couldn't find any research that
tested that reaction in people. So she decided to set up the first experiment to study
lactation aggression in humans.
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 women's infants were supervised in an
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 breastfeeding 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 breastfeeding mothers nursed their
The researchers also measured participants' stress levels via blood pressure during
the experiment. breastfeeding 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 breastfeeding mothers during acts of aggression, the researchers say, is an
indication that the same mechanism is likely at work in humans as well.
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.