Mama Bear: Ever wonder whether breastfeeding changed your behavior?

This week’s article highlights many of the fun things we do in psychology research. Of course, it started with a question:

Does breastfeeding increase aggression?

Many lactating animals (e.g., mice, rats, prairie voles, hamsters, lions, domestic cats, rabbits, squirrels, and domestic sheep) are more aggressive than their non-lactating peers. In animals, lactation increases aggressive behavior, and reduces physiological responses to stress. This makes sense because for many animal, and even humans until 100 years ago, many infants didn't survive. Lactation is a way for the mother's body to know that there is an infant to still take care of. However, humans have built societies and homes, invented seatbelts, and established food protection standards. So, it remains to be documented whether human women become more aggressive after having a baby, and what physiological processes support this change in behavior.

To answer this question, Dr. Jennifer Hahn-Holbrook, now a professor at Chapman University, and her colleagues conducted a very cool experiment to understand whether breastfeeding women are more aggressive, and whether that aggression is accounted for by reduced physiological responses to stress.

To do this, they recruited three groups of women: 20 women with infants between 3-6 months who were exclusively breast-feeding, 20 women with infants between 3-6 months who were feeding their infant a mix of breast milk and formula, and 20 women who had never had a baby. The women came to the laboratory and met what we call a “confederate.” A confederate is a person introduced to the participants as another participant, but in actuality is part of the experiment. The confederate was trained to be rude. Rude behavior involved ignoring the actual participant, chewing gum, and checking their cell phone during the experiment instructions. This was intended to cause the actual participants to make what psychologists call fundamental attribution error, or the tendency to place an undue emphasis on internal characteristics (personality) to explain someone else’s behavior in a given situation rather than considering the situation’s external factors. For example, when you cut someone off you know it’s because you are late and would otherwise miss your exit. When someone else cuts you off, it’s because they are a jerk.

The participants were then instructed to play a game against the other “participant” that involved reaction time. Fastest person to respond wins. The winner of each round got to push a button that released a loud sound at the loser. The participant controlled the volume (up to 150 decibels) and duration (up to 5 seconds) of the sound with their button push. After 8 rounds of the game, participants fed their babies (breastfed, formula fed) or took a break if they were in the non-mother control group. Then, the participants played the game again for another 8 rounds. The mothers’ blood pressure was monitored and recorded throughout the entire procedure.

They found that breastfeeding mothers delivered longer and louder aversive sounds to their rude competitors, compared to both the mixed-feeding mothers and the non-mothers. They also found that breastfeeding women had smaller increases in blood pressure (a measure of stress) while playing the competitive game than women in either of the other groups.

The research team concluded by saying that women who are breastfeeding, not just new moms, were more likely to be aggressive than women who are not. Supporting this idea, they found that there was a significant positive correlation between aggression during the game and the percent of the infant’s diet that was breastmilk (as a measure of how much the mother is lactating). Therefore, they believe that the physiology that supports lactation in new moms supports aggressive behavior that is protective to the infant. And further, lactating mothers show a reduced response to stress, which likely helps facilitate aggression in times of threat and competition.

So, what can we take away from this? I think on a very basic level, knowledge is power. If you or someone you know is breastfeeding, they are likely to be more aggressive than they are usually, and more aggressive than other people. This is normal and adaptive. Don’t hold it against them or yourself. Aggression has many different dimensions. In this study it was the force with which women delivered aversive sounds to competitors, like a punishment. But what might this behavior look like outside of the laboratory? Potentially, it’s perceiving other people as a threat, competing with other moms for no apparent reason, delivering unusually harsh punishments to your partner or older children for potential threats to the new baby. Just remember, increases in aggression are normal, and being driven by the many, many changes to your physiology that allowed you to have the baby and care for it in the first place.

In these modern times, we take for granted all of the physiological changes that come with being able to sustain a pregnancy, have a baby, and breastfeed that have been promoting human survival for thousands of years. As a result, many women don’t learn about these physiological changes until they are going through it, and society doesn’t do much to support them. Don’t be part of that problem.

Hahn-Holbrook, J., Holt-Lunstad, J., Holbrook, C., Coyne, S.M., & Lawson, E.T. (2011). Maternal defense: Breast feeding increases aggression by reducing stress. Psychological Science. DOI: 10.1177/0956797611420729