Testicular volume is inversely correlated with nurturing-related brain activity in human fathers

Abstract

Despite the well-documented benefits afforded the children of invested fathers in modern Western societies, some fathers choose not to invest in their children. Why do some men make this choice? Life History Theory offers an explanation for variation in parental investment by positing a trade-off between mating and parenting effort, which may explain some of the observed variance in human fathers' parenting behavior. We tested this hypothesis by measuring aspects of reproductive biology related to mating effort, as well as paternal nurturing behavior and the brain activity related to it. Both plasma testosterone levels and testes volume were independently inversely correlated with paternal caregiving. In response to viewing pictures of one's own child, activity in the ventral tegmental area--a key component of the mesolimbic dopamine reward and motivation system--predicted paternal caregiving and was negatively related to testes volume. Our results suggest that the biology of human males reflects a trade-off between mating effort and parenting effort, as indexed by testicular size and nurturing-related brain function, respectively.

Keywords: empathy; sperm competition.

Fig. 1.

Relationship between reproductive biology and paternal investment. (A) Caregiving vs. testes volume residuals after testes volume was regressed against height. The dotted line indicates the score (72) at which mothers and fathers are equally responsible for their child’s daily care. Scores below 72 imply that the mother does more than the father and scores above 72 imply the opposite. (B) Caregiving vs. plasma testosterone levels. (C) Plasma testosterone levels vs. testes volume residuals (regressed against height).

Fig. 2.

Relationship between brain function, paternal behavior, and reproductive biology. (A) Main effect of the contrast Own Child–Adult for all emotions combined, Bonferroni-corrected P < 0.001, uncorrected P < 1.04 × 10⁻⁸. (B) Plot of caregiving vs. β-contrast values Own Child–Adult from functionally derived VTA ROI. The dotted red line indicates the score (72) at which mothers and fathers are equally responsible for their child’s daily care. (C) Plot of β-contrast values Own Child–Adult from VTA ROI vs. residual testes volume (regressed against height and testosterone).

Fig. 3.

Functionally derived ROI in the VTA/substantia nigra (green) overlaid on the results of a whole-brain exploratory analysis using testes volume as a covariate (blue). Results are thresholded at P < 0.001, uncorrected.