I am currently writing a trade book on my work at the nexus of evolutionary theory and consumer behavior. The book will be in print next year. I hope that you'll check it out. In conducting some research on the consumption of pornography (click here for one of my recent posts on the topic), I came across an article coauthored by Sarah J. Kilgallon and Leigh W. Simmons published in Biology Letters wherein they reported on what I thought was an incredible finding.
Men were asked to view one of several sexual images that vary in the extent to which they trigger sperm competition. For example, an image of two men having sex with one woman is inherently associated with sperm competition whereas one wherein three women are having sex with one another involves no sperm competition. The men masturbated as they viewed their assigned image, and were then required to drop off their semen at the lab. The researchers were interested in seeing whether images that cue sperm competition would yield sperm of higher quality, as well as yielding a greater quantity of sperm. Incredibly, the sperm motility (captures how well sperm navigates toward the ovum) of men who viewed the "sperm competition" image was higher than that of men who had watched the "non sperm competition" image. This held true between-subjects and within-subjects (but only when the order of presentation was sperm competition image first followed by the image of non sperm competition). Interestingly, the results for sperm density yielded the opposite effect. In other words, men who viewed the "sperm competition" images have a lower density of sperm compared to their "non sperm competition" counterparts.