Self-Deception about Fecundity in Women

Abstract

Concealed fecundity and extended female sexual receptivity have evolved in some primates, including humans, conferring advantages both within primarily monogamous relationships (e.g., benefits from paternity assurance) and from extrapair liaisons (e.g., better access to good genes). As humans evolved the intellectual capacity for decision-making, women became capable of altering their own fertility. In some circumstances, they may choose to ameliorate risks and responsibilities associated with pregnancy by reducing sexual motivation near the perceived most fecund time of their menstrual cycle. But three findings—a general inability of women to accurately recognize their own intervals of fecundity, high variability in ovulation timing, and unconscious transmission and reception of cues associated with fecundity—constitute a physiological and behavioral syndrome that can be considered self-deception. In this study, I develop a descriptive model to determine implications of the hypothesis that these features of female and male physiology and behavior have been shaped by natural selection in response to female decision-making. My analysis shows that consensus motivation for coitus between partners influences both the importance of variable ovulation date and the probability of conception, under the influence of self-deception. It also identifies priorities for future empirical work.

Appendix: Finding the Expected Level of Female Desire

Since female desire is generally variable over the menstrual cycle, we must find the expectation over the cycle (D_F), based on the frequencies of ovulation times and desire magnitudes associated with each. (We assume that male desire remains constant at D_M = m_x during the fecundity window.) We can obtain the expected female desire at ovulation time x_o (D_O(x_o)) analytically for the piecewise quadratic female desire function as follows:

$$ {\displaystyle \begin{array}{l}{D}_1\left({x}_o\right)=\frac{1}{f_w}{\int}_{x_o-{f}_w-{x}_m}^{x_o-{x}_m}{D}_{Q1}\left(x-{x}_m\right) dx=\frac{1}{f_w}\left\{{f}_x{f}_w-\frac{\left({f}_x-{f}_n\right)\left[{\left({x}_o-{x}_m\right)}^2-{\left({x}_o-{f}_w-{x}_m\right)}^2\right]}{x_n-{x}_m}+\frac{\left({f}_x-{f}_n\right)\left[{\left({x}_o-{x}_m\right)}^3-{\left({x}_o-{f}_w-{x}_m\right)}^3\right]}{3{\left({x}_n-{x}_m\right)}^2}\right\},\mathrm{for}{x}_o<{x}_n;\\ {}{D}_2\left({x}_o\right)=\frac{1}{f_w}{\int}_{x_o-{f}_w-{x}_m}^{x_n-{x}_m}{D}_{Q1}\left(x-{x}_m\right) dx+\frac{1}{f_w}{\int}_0^{x_o-{x}_n}{D}_{Q2}\left(x-{x}_n\right) dx=\frac{1}{f_w}\left\{{f}_x\left({x}_n+{f}_w-{x}_o\right)-\frac{\left({f}_x-{f}_n\right)\left[{\left({x}_n-{x}_m\right)}^2-{\left({x}_o-{f}_w-{x}_m\right)}^2\right]}{x_n-{x}_m}+\frac{\left({f}_x-{f}_n\right)\left[{\left({x}_n-{x}_m\right)}^3-{\left({x}_o-{f}_w-{x}_m\right)}^3\right]}{3{\left({x}_n-{x}_m\right)}^2}\right\}\\ {}\begin{array}{l}+\frac{1}{f_w}\left\{{f}_n\left({x}_o-{x}_n\right)-\frac{\left({f}_i-{f}_n\right){\left({x}_o-{x}_n\right)}^3}{3{\left({x}_c-{x}_n\right)}^2}\right\},\mathrm{for}{x}_n<{x}_o<{x}_n+{f}_w;\mathrm{and}\\ {}{D}_3\left({x}_o\right)=\frac{1}{f_w}{\int}_{x_o-{f}_w-{x}_n}^{x_o-{x}_n}{D}_{Q2}\left(x-{x}_n\right) dx=\frac{1}{f_w}\left\{{f}_n{f}_w+\frac{\left({f}_i-{f}_n\right)\left[{\left({x}_o-{x}_n\right)}^3-{\left({x}_o-{f}_w-{x}_n\right)}^3\right]}{3{\left({x}_c-{x}_n\right)}^2}\right\},\\ {}\mathrm{for}\ {x}_o>{x}_n+{f}_w.\end{array}\end{array}}\kern0.75em $$

(6)

The consensus desire function D is in text eq. (3). For each ovulation interval x_o + dx_o, the chance of conception during f_w is

$$ {P}_C={F}_O\left({x}_o\right)\left(1-{e}^{-\upvarphi D\left({x}_o\right){f}_w}\right)d{x}_o. $$

(7)

But because D(x_o) is defined piecewise, this function must be integrated accordingly to find the probability of conception over a cycle as

$$ {P}_C=\left\{\begin{array}{c}{\int}_{x_{m+{f}_w}}^{x_n}{F}_O\left({x}_o\right)\left(1-{e}^{-\upvarphi {D}_1\left({x}_o\right){f}_w}\right){dx}_o+{\int}_{x_n}^{\left({x}_n+{f}_w\right)}{F}_O\left({x}_o\right)\left(1-{e}^{-\upvarphi {D}_2\left({x}_o\right){f}_w}\right){dx}_o\\ {}+{\int}_{x_n+{f}_w}^{x_c}{F}_O\left({x}_o\right)\left(1-{e}^{-\upvarphi {D}_3\left({x}_o\right){f}_w}\right){dx}_o\end{array}\right\}. $$

(8)

Assuming 12 cycles per year, the probability of conception over a year is then

$$ {P}_Y=1-{\left(1-{P}_C\right)}^{12}. $$

(9)