It's an age-old story. A heterosexual couple is having struggles regarding their intimate lives. Quite often, a man might complain that the relationship is too low on the sex meter. On the other side of the table, the woman may well be complaining that the relationship is too low on the love meter.

In multiple strands of research, sex differences emerge when it comes to the psychology of sex and love. And these differences can be explained with an evolutionary perspective (see Geher & Kaufman, 2013). The dramatically different reproductive systems between men and women seem to map importantly onto differences in the psychology of sexuality and love.

Women's reproductive systems are complex and intensive. To successfully reproduce on one occasion, a woman must invest an enormous amount of time and energy (with pregnancy only being a slice of it) to succeed. Whereas for men, one sex act alone is sufficient for successful reproduction. This asymmetry in parental investment (see Trivers, 1972) helps to understand asymmetries between the sexes in relationships.

Famously, women, more than men, tend to connect love and sexuality. Evidence on this point comes from various lines of research. One includes research on sex differences in sociosexuality, or the tendency to be OK with sex outside of the bounds of committed, loving relationships. Consistently, men tend to score higher (on average) than women on this dimension (see Gangestad & Simpson, 1990; Moreno & McKerral, 2015)—suggesting that men connect sex and love less than women do.

A deep way to understand this sex difference is through the lens of parental investment theory (Trivers, 1972), as described above. For a woman to successfully reproduce, she not only needs to get pregnant and successfully deliver a baby—she typically also needs to feed the baby, provide shelter and safety, etc. For her, the baby is a huge investment of everything that she has. If she has a partner to help her care for the baby and provide resources for the family, the baby is more likely to thrive compared to otherwise. A great emotional way to maintain........

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