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Unlike some species, humans survive by having sex—and many adult humans are happy to fulfill their biological imperative. But the costs (and risks) associated with being a two-sexed species are pretty steep, notes researcher David Bishai, MD, PhD, MPH, associate professor of Population, Family and Reproductive Health.

He estimates that the average American spends between $1,000 and $8,700 yearly on activities related to sex—the preambles, the act itself and the fallout.

So why not just take our cues from whiptail lizards and reproduce asexually—and on the cheap? Because the benefits of sexual reproduction outweigh the costs.

With two parents, humans are able to pool more diverse combinations of genes, making us more adaptable and more resistant to pathogens than, say, bdelloid rotifers. It makes us more likely to evolve. And, according to Bishai, that genetic diversity (and, some would add, pursuit of pleasure) is the payoff for the very expensive human sex drive.