Mating Fast and Slow?

– By Tran Dinh and Steve Gangestad

Why do humans vary in their willingness to have multiple sexual relationships or tendency to form long-term bonds with a single partner? Evolutionary-minded researchers often look at patterns observed in nonhuman animals to gain insight into human nature. Indeed, much research has been done to explain the array of mating strategies across and within species: why individuals engage in intense mating contests, display exaggerated physical characteristics, cheat on their partners—and also why they form monogamous bonds or mate with many partners.

Over the past few decades, some scholars have argued that variations in human sociosexuality—their willingness to have sex without commitment or interest in having multiple sex partners—are part of fast vs. slow life history strategies. Across species, those that are fast life history strategists reproduce at an early age, have many offspring, and die young. Slow strategists invest in their health, have offspring late, have few of them, and live long lives. Within humans, fast and slow life history strategies may exist too. According to dominant views, humans achieve fast or slow reproductive outcomes by having uncommitted sex with multiple partners (unrestricted sociosexuality, a “fast” strategy) or settling down with one committed partner and having few lifetime sex partners (restricted sociosexuality, a “slow” strategy).

At first blush, it may seem a compelling argument. More sex leads to more offspring, right? Less commitment leads to more sex partners which leads to more offspring. The logic sparked much research into life history trajectories explaining not just variations in sociosexuality, but also personality traits and aggression. Here, we focus just on the question of whether restricted vs. unrestricted sociosexuality solves the adaptive problem of reproducing at a slow vs. fast rate. Before we get into addressing that question (spoiler: the answer is no), it is important to first discuss life history theory.

Life History Theory and the Fast-Slow Continuum

            The life histories and life cycles of animals vary tremendously. The giant sunda rat lives less than a year, the bowhead whale over 200 years. Yet both grow, develop, and successfully reproduce within their lifespans. How has natural selection shaped such a vast diversity of life cycles? How can it pay members of one species to spend more time gestating a single offspring than members of another species spend living? Life history theory seeks to answer these questions.

The theory argues that species’ life courses vary along a fast-to-slow dimension. Fast species tend to reproduce early in life. They put a lot of effort into reproduction, which means that they have less to invest in fighting infections, suppressing cancer, avoiding or defending against predators, and other tactics for extending their life. Slow species start reproducing later in life. They instead prioritize growth and development. Their strategy is to put more effort into living longer, which comes at the expense of reproducing more slowly.

The “winning” strategy partly depends on the cost that members of a species must pay to avoid death. When the chances of dying young are low or can be reliably avoided, reproducing slowly across a longer lifespan—and “helping” each offspring survive and reproduce—becomes more profitable. But waiting to reproduce when the world is sufficiently dangerous risks not reproducing at all. Reproducing early and plentifully—but not investing heavily in any one offspring—becomes a better strategy.

These differences between species sparked the idea that life history strategies may differ within a species too. Humans may also differ in the speed that they grow, reach puberty, have kids, and age. These differences may arise from growing up in an environment with high chances of relatively uncontrollable death, or in a relatively safe controllable environment.

Sociosexuality as Part of a Fast vs. Slow Strategy

            So far, these ideas are conceptually plausible. But there is an additional element to dominant viewpoints—that sociosexual orientations underlie fast vs. slow reproductive strategies. Fast female strategists are purportedly more open to mating with multiple males (serially or concurrently) in relatively uncommitted or transient relationships. Their unrestricted sociosexuality or “short-term” mating strategy serves the purpose of reproducing sooner, having more offspring, and caring less for each child. In contrast, slow female strategists desire strong, stable romantic relationships and are less open to having uncommitted sex. Their restricted sociosexuality or “long-term” mating strategy allows them to reproduce later and within a lasting relationship in which both partners intensively care for fewer offspring.

For multiple reasons, we question how well these proposed links are theoretically grounded.

First, pair-bonding does not systematically associate with fast-slow life history variation across species. Bats have amongst the slowest life histories of any mammal. Yet female bats mate with multiple males and rear young without male assistance. Among bird species, some slow strategists, such as albatrosses, pair-bond for life. But so do some relatively fast strategists, including many geese and swans. Similarly, primate species that pair-bond do not systematically have slower life histories than those that do not pair-bond.

Second, a reason for these patterns is that pair-bonding and biparental care do not promote offspring quality at the expense of offspring quantity. Females of bird species that pair-bond and engage in biparental care have more offspring than female birds that do not have male assistance. Female marmosets and tamarins—primates that raise offspring in pairs or small family groups—have relatively high rates of reproduction among primates.

Third, reflection of how humans have evolved (e.g., as cooperative breeders) offers little basis for thinking that female “fast mating” and having many unstable sexual relationships promote the speed that offspring are produced. Human offspring require investment across nine months of pregnancy. In foraging societies, mothers breastfeed children for a year or more. Having additional sex partners during this time does not increase how fast a woman has children. Having sex with one man or several men may result in one child. Indeed, fidelity to a single partner, who helps provide nutrition or care for children, may best promote the rate that women can have children. Across foraging societies, how much men provide food negatively covaries with the spacing between births and the number of children women have.

The Take-Home

It is plausible that variation in rates of growth and development, such as when girls have their first menstrual period, partly reflects differences in life history strategies. Mating propensities likely systematically vary as well, as reflected in differences in tendencies to form stable pair-bonds or willingness to have multiple sexual relationships. However, there is little compelling reason to think that variations in sociosexuality explain differences in reproductive rate or offspring quality vs. quantity in humans—or, for that matter, across nonhuman animal species. What may appear to be a logical link is not supported by evolutionary thinking or pertinent data. Our understanding of both life history variations among humans and variations in mating propensities will benefit from seeing them as largely unrelated phenomena.

Read the full article here: Dinh, T., & Gangestad, S.W. (2024). Mating fast and slow? Sociosexual orientations are not reflective of life history trajectories. Evolution and Human Behavior, 45(1), 27-40.