How males and females differ is interesting, in a timeless way. The study of biological sex and gender differences are vibrant research disciplines.
Sex differences frequently make the news (just google “Larry Summers” if you need convincing on that one) and are popular topics for books. I’ve even blogged about sex-related structural differences in the brain.
Despite profound interest in how males and females are different, animal research skews towards males. The majority of animal subjects — whether they be rodents or nonhuman primates — are males.
Why? Because the post-pubescent female brain is in a constant state of hormonal flux! Hormones can affect brain structure and function as well as behavior (and of course mood).
Charlotte Boettiger’s lab at the University of North Carolina studied how decision-making differed in females at during their menstrual cycle. The experiment used intertemporal decisions, or choices between an immediately available reward and delayed, or future, reward.
In an intertemporal decision, the reward available now is less than the delayed reward. An example intertemporal decision is: “Would you like $10 today or $12 in one week?”
People take time into account, and the subjective value of the delayed reward is less than the objective value. This change in value for delayed rewards is called delay discounting; delay discounting happens in other animals and for different kinds of rewards.
Participants in the study (a total of 91) were naturally and regularly menstruating females. All women filled out self-reports about the timing of their ovarian cycles, but in a subset of participants (34 women), the researchers measured the hormone estradiol in saliva samples. Estradiol peaks immediately preceding ovulation, when fertility is greatest.
In general, females discount delayed rewards less than males. Females are more likely to forego an immediate reward in favor of the greater, delayed reward.
This study found females’ preference for immediate rewards varied with the ovarian cycle and was weakest when fertility peaked. When fertile, females discounted future rewards less, increasing the subjective value of those future rewards.
Levels of the hormone estradiol tracked discounting in females too.
Women with a measurable increase in estradiol showed less bias towards immediately available rewards than those without measurable changes in hormone levels.
The researchers did not just measure decision-making behavior and hormones. They linked both behavior and estradiol to genes associated with dopamine levels in the prefrontal cortex. Both the dopamine system and prefrontal cortex are important players in how the brain values rewards and makes decisions, and this study suggests an important .
I blogged about this study because in my family, we females like to blame everything on hormones. Sometimes we do it seriously but frequently just in jest.
Studies like this one show that we play the blame game pretty well; hormones do have an important impact on both brain and behavior.