Extended Data Fig. 1: Double ovulation as a threshold trait.

a, Three conditional (age-dependent) ovulatory strategies that differ in age of switching from single (dashed line) to double ovulation (solid line). b, Threshold trait depiction of a cohort of population with normally distributed variation in age at switching (mean age at switching from single to double ovulation is 30 yrs., SD of 3 yrs.). At age 27, the threshold (vertical line positioned at age 27) divides the population into individuals that double ovulate (those with switching ages less than 27) and single ovulate (those with switching ages greater than 27). c, As age increases, the fraction of double ovulators increases as a cumulative normal function as the threshold in b shifts to the right, increasing the fraction of the distribution of switching ages that is less than the threshold.