part of the brain showed a greater response when sharing money, while in men, the same structure showed more activity when
they kept the cash for themselves, a small study
published Monday in Nature Human Behavior found.
Women tend to be more altruistic than men, previous studies have shown.
As Philippe Tobler, co-author of the new study, sees it, "women put more subjective value on prosocial behavior and men find selfish behavior more valuable."
"However, it was unknown how this difference comes about at the level of the brain," Tobler, an associate professor of neuroeconomics and social neuroscience at University of Zurich, wrote in an email. "But in both genders, the dopamine system encodes value."
By "encode," he means the activity in our brain changes in proportion to the value we give social experiences.
Searching for answers for why women and men are not equally selfish, he and his colleagues focused on the dopamine system.
Dopamine, which plays a fundamental role in the brain's reward system, is released during moments of pleasure, yet it also helps us process our values. This mental ability transpires within the brain machinery known as the striatum. Latin for "striped," the striatum is threaded with fibers that receive and transmit signals from the cerebral cortex, the thalamus and other brain regions.
Tobler and his colleagues designed a series of experiments to test how dopamine might influence the behavior of men and women. Fifty-six male and female participants made choices between sharing a financial reward with others or keeping the money for themselves.
Given only a placebo before making decisions, women acted less selfishly than men, choosing to share their money with others. However, when their dopamine systems were disrupted after they received a drug called amisulpride, women acted more selfishly, while men became more generous. Amisulpride is an antipsychotic normally used to treat the symptoms of schizophrenia.
"Based on the opposing priorities of the genders, interfering with the dopamine system has opposing effects," Tobler said.
In a second experiment, the researchers used functional MRI to investigate changes in the brain while eight female and nine male participants made choices. Compared with the males, the striatum in females showed more activity when they made a prosocial decision.
According to Anne Z. Murphy, an associate professor of neuroscience at Georgia State University, other research
has shown "that females are more prosocial. We find it more rewarding, and if you manipulate dopamine signaling in the brain, you can make females less prosocial and males less selfish." Murphy was not involved in the study.
Still, she said, the study brings "greater awareness to the fact that there are brain differences in male and females."
"It just shows, once again, that people can point to a biological basis for some of the characteristics that are prototypically male," Murphy said. These traits would include selfishness, self-promotion, generally, a hard-driving profile.
"Now, you can point to another biological basis for it," she added, "And rather than using this knowledge to divide us, maybe we can use this to help make society a better place."
For instance, she said, when women act in more altruistic ways, they shouldn't be regarded as less deserving than male colleagues who are more self-promoting.
Gender differences in the brain may not be due to structural differences -- for example, variations in region size or shape based on sex, noted the researchers. Gender differences in the brain could be functional. This would mean a flood of the very same neurotransmitter -- dopamine -- might cause a very different response in women than in men.
"It may be worth pointing out that the differences are likely to be learned," Tobler said.
Though male and female tendencies may be learned, Murphy said, these behaviors are not acquired in a single lifetime.
'Shaped by history'
Instead, these preferences develop over time based on the differing roles of females and males: "reproduction versus resource-gathering," Murphy said.
"You see similar behavior in rodents," she said, noting that female rats act in more altruistic ways than males. "It's evolutionarily conserved. It's shaped by history."
The study has implications for drug research, Tobler noted.
"Historically, medical drugs were often tested primarily on men and sometimes drugs have been found to be more effective in men than women," he wrote.
Murphy explained that "preclinical studies have shown that females require approximately twice the amount of morphine than males to produce the same level of analgesia."
All opiates that are metabolized in one specific way produce what is known as a "sexually dimorphic response," she added.
"People are starting to look at whether cannabinoids are sexually dimorphic. It's been suggested that cannabinoids are more effective in females than in males," she said, with a lot of preclinical data showing this is the case.