Ovarian cycle effects on immediate reward selection bias in humans: A role for estradiol

A variety of evidence suggests that, among humans, the individual tendency to choose immediate rewards ("Now") over larger, delayed rewards ("Later"), or Now bias, varies with frontal dopamine (DA) levels. As cyclic elevations in estradiol (E) modulate other frontal DA-dependent behaviors, we tested ovarian cycle effects on Now bias, and whether any such effects are E mediated. To do so, we quantified Now/Later choice behavior in naturally cycling adult females (n = 87; ages 18-40 years) during both the menstrual phase (MP; cycle day 1-2; low E), and the follicular phase (FP; cycle day 11-12; high E). Now bias decreased an average of 3.6% from MP to FP (p = 0.006). Measures of salivary E levels at each visit were available in a subsample of participants (n = 34). Participants with a verified E rise from MP to FP showed significantly greater decreases in Now bias at mid-cycle (n = 23) than those without a rise (n = 11; p = 0.03); Now bias decreased an average of 10.2% in the E rise group but increased an average of 7.9% in the no E rise group. The change in Now bias from MP to FP inversely correlated with the change in E (p = - 0.39; p = 0.023), an effect driven by individuals with putatively lower frontal DA based on genotype at the Val¹⁵⁸Met polymorphism in the COMT gene. This is the first demonstration that intertemporal choice varies across the ovarian cycle, with Now bias declining at mid-cycle, when fertility peaks. Moreover, our data suggest that the interacting effects of estradiol and frontal DA mediate this cycle effect on decision making.