Original ArticleMating strategy flexibility in the laboratory: Preferences for long- and short-term mating change in response to evolutionarily relevant variables
Introduction
Humans exhibit a great diversity of sexual behaviour. Some people only mate within a pair-bond, while others prefer casual sex with partners to whom they have little emotional attachment. Worldwide, monogamy is the most common mating arrangement. However, the number of strictly monogamous cultures is fairly small. Of the 1231 cultures included in the Ethnographic Atlas (Gray, 1998, Murdock, 1967), only 15.1% (n = 186) practice monogamy exclusively while 84.6% (n = 1041) allow a man to have more than one wife. There is even a small number of societies (n = 4) where a woman is able to have more than one husband.
What underlies this variation in human mating behaviour? In some species, including many arthropods, mating strategies are strongly linked to specific sets of alleles (Shuster, 2010, Shuster and Wade, 2003), and thus are generally fixed from conception. In other species, including some toads, individuals “choose” a mating strategy early in life in response to variables such as body size (Leary et al., 2005, Moczek and Nijhout, 2002, Simpson et al., 2011). In mammals, at least some plasticity in mating behaviour seems to be retained throughout the lifespan (Eberle and Kappeler, 2004, Koprowski, 1993, Schradin et al., 2010). The focus of the present research is to investigate the extent to which humans exhibit such plasticity.
According to many evolutionary psychologists, individual differences in mating behaviour can be explained in part by the differential implementation of mating strategies – sets of psychological adaptations which have evolved to help individuals negotiate the many tasks involved in successful mating. Two main theories have applied the mating strategy perspective to humans. The first is Sexual Strategies Theory (SST; Buss & Schmitt, 1993); the second is Strategic Pluralism Theory (SPT; Gangestad & Simpson, 2000). In their article introducing SST, Buss and Schmitt identified two main modes of mating in our species: long-term and short-term. Individuals pursuing a long-term strategy tend to mate with one partner at a time within a committed relationship, while those who pursue a short-term strategy are less interested in sexual exclusivity or commitment. According to SST, both strategies could sometimes boost fitness in the ancestral environment, and both were associated with their own adaptive problems (e.g., detecting commitment in a potential long-term partner). A key component of the theory is the idea that humans may adaptively shift their mating strategies, depending on the circumstances.
Seven years later, Gangestad and Simpson expanded on the idea that human mating strategies are deployed conditionally using models from behavioral ecology. According to their SPT, the costs and benefits of a long- vs. short-term mating strategy vary from person to person depending on circumstances, and the strategy a given individual pursues will change accordingly.
For example, ceteris paribus, a man in a society with many available sexual partners (and thus low intrasexual competition), might find himself able to establish low-commitment relationships with several women. However, in a society with few available sexual partners (and thus high intrasexual competition), the same man might find himself unable to establish any low-commitment relationships at all. Thus, there lies a switch-point at which the fitness benefits of pursuing one partner for a long-term relationship is more beneficial than pursuing multiple partners for casual relationships. It is at this point that we would expect men to switch from pursuing a short-term mating strategy to a long-term one. This example focuses on a single variable, namely partner availability. According to SPT, many variables jointly determine which strategy a given individual adopts. These include the individual's attractiveness, social standing, and mating opportunities.
Together, SST and SPT provide a framework which explains what mating strategies are, why they evolved, and how they might be activated. At the same time, though, the theories say relatively little about which circumstances may cause a strategy shift and when this might happen. There are several possibilities. One is that humans have a critical period for mating strategy selection, perhaps during adolescence, in which they develop a permanent strategy preference in response to environmental input. A second possibility is that shifts in strategy remain possible throughout the lifespan, but occur only in response to substantial life events (e.g., a marked change in social status). A third and final possibility is that men and women respond rapidly to new mating-relevant information in their environment, shifting the relative strength of their long- and short-term mating inclinations within relatively brief windows of time. The importance of this question has been highlighted by the researchers behind SST and SPT. According to Buss (2002), for instance:
Further research is needed on the context-sensitive nature of human mating strategies. Precisely which circumstances might cause a person to shift from a long-term mating strategy to a short-term mating strategy and vice-versa? Which circumstances might trigger an extramarital affair, or conversely, cause someone to forgo an alluring sexual opportunity? (p. 57).
Similarly, Gangestad and Simpson (2000) note that ‘Conditional strategies are a central topic in behavioral ecology... However, the possible role of conditional strategies in human mating has received relatively little attention’ (p. 578). Seventeen years later, there is still comparatively little research addressing this question.
To be fair, a number of studies address the question indirectly. This includes both correlational and experimental research. Correlational research suggests that men and women's mating strategies are calibrated to their personal attributes. Lukaszewski, Larson, Gildersleeve, Roney, and Haselton (2014), for example, found that short-term mating behaviours positively correlate with physical strength in men and attractiveness in both sexes. Unfortunately, such studies do not tell us whether men and women recalibrate their strategies in response to changes in their circumstances.
Experimental research, in contrast, indicates that partner preferences can be manipulated within the laboratory. Little and colleagues, for instance, showed that preferences for facial symmetry and masculinity can be modulated by cues of intra-sexual competition and pathogen presence (Little et al., 2007, Little et al., 2011, Little et al., 2013), although see Li et al. (2014). Similarly, Welling et al. (2007) showed that increases in women's circulating testosterone heighten their preference for facial masculinity. Finally, some studies suggest that behavioral traits plausibly linked with different mating strategies can be manipulated. Wilson and Daly (2004), for example, found that men become more impulsive following exposure to images of attractive women, while women become more impulsive following exposure to cues of wealth. What these experiments have in common is that they focus on a single attribute associated with either short- or long-term mating (e.g., specific partner preferences or characteristics such as impulsivity), rather than directly measuring changes to a person's strategy orientation. It thus remains uncertain whether these effects are limited to the specific variable of interest, or represent a more general shift in mating strategy.
To address this research gap, we developed a task to measure people's inclination towards short- and long-term mating. We then ran a series of experiments in which the task was completed twice, once to get a baseline reading, and a second time after a priming intervention/manipulation. We ran three experiments in total, each one priming a different context thought to have affected the success of long- and short-term mating in ancestral humans. These contexts were parental care, resource-abundance, and danger.
Section snippets
Experiment 1: parental care
Human beings have evolved large brains but a narrow pelvic girdle (necessary for bipedal locomotion). This has had a large impact on childbirth. Compared to other primates, our offspring are born in a markedly underdeveloped state, so that they can fit through their mother's pelvis (Wittman & Wall, 2007). As a result, human neonates are fully dependent on the care and attention of older humans for several years. One of the reasons humans evolved to form durable pair-bonds was to provide a
Experiment 2: resource-abundance
One of the hypothesized benefits of pair-bonded mating in ancestral humans was that it allowed for the efficient acquisition of resources for highly dependent offspring. Compared to a single parent, those cooperating within a pair-bond would have been able to divide responsibilities and each specialise in a subset of tasks (Geary, 2000, Quinlan, 2008, Winking et al., 2007). Furthermore, a pair-bond allows for one partner to compensate for the other when they find it harder to provide for the
Experiment 3: physical danger
In the ancestral environment, women and their dependant offspring would often have been vulnerable to predators (Hart and Sussman, 2009, Prokop and Fancovicova, 2010) and other human groups (Navarrete et al., 2009). In such circumstances, establishing a pair-bond with a man willing to protect his mate and her offspring would likely have enhanced women's fitness. At the same time, ancestral men would have increased their fitness by protecting their offspring and mates from such threats.
The
General discussion
Overall, we found moderate support for our hypotheses. As predicted, the parental care primes in Experiment 1 caused an increase in long-term relationship interest, and the resource-abundance primes in Experiment 2 increased short-term relationship interest. In Experiment 3, danger primes increased long-term mating interest, which was also in line with our predictions, though we also found an unexpected increase in short-term mating interest. We also hypothesized, in every experiment, that
Acknowledgements
We would like to thank Lisa Davies, Katheryn Egleton, Leanne Francis, Romi George, Sera Griffiths, and Isabel Turner for their help with data collection. We would also like to thank the two anonymous reviewers of our original manuscript for their insights and helpful suggestions which we used to improve this paper.
Declaration of interest
Conflicts of interest: none.
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