Short-term mating was unrelated or even negatively related to reproductive success; long-term mating predicted a greater number of children and children's children

Phenotypic Signals of Sexual Selection and Fast Life History Dynamics for the Long-Term but Not Short-Term Mating. Janko Međedović. Evolutionary Psychology, November 29, 2021. https://doi.org/10.1177/14747049211057158

Abstract: Mating patterns are crucial for understanding selection regimes in current populations and highly implicative for sexual selection and life history theory. However, empirical data on the relations between mating and reproductive outcomes in contemporary humans are lacking. In the present research we examined the sexual selection on mating (with an emphasis on Bateman's third parameter – the association between mating and reproductive success) and life history dynamics of mating by examining the relations between mating patterns and a comprehensive set of variables which determine human reproductive ecology. We conducted two studies (Study 1: N = 398, Study 2: N = 996, the sample was representative for participants’ sex, age, region, and settlement size). The findings from these studies were mutually congruent and complementary. In general, the data suggested that short-term mating was unrelated or even negatively related to reproductive success. Conversely, long-term mating was positively associated with reproductive success (number of children in Study 1; number of children and grandchildren in Study 2) and there were indices that the beneficial role of long-term mating is more pronounced in males, which is in accordance with Bateman's third principle. Observed age of first reproduction mediated the link between long-term mating and number of children but only in male participants (Study 2). There were no clear indications of the position of the mating patterns in human life history trajectories; however, the obtained data suggested that long-term mating has some characteristics of fast life history dynamics. Findings are implicative for sexual selection and life history theory in humans.

Keywords: short-term mating, long-term mating, fitness, reproductive ecology, sexual selection, life history theory

General Discussion

Variation in mating behavior is certainly one of the crucial determinants of variance in fitness itself. Interestingly, the empirical data on the associations between mating and reproduction as a prerequisite for the analysis of selection regimes acting on mating, including sexual selection, are surprisingly lacking, especially in industrial and postindustrial human populations. This topic is of high importance, not only from the viewpoint of sexual selection, but life history theory in humans as well, together with the potential demographic implications. In order to explore the role of mating in reproductive ecology we conducted two studies with samples which differ in important reproductive characteristics (including the mean age of participants in two samples) and assessing different outcomes related to the environment and reproductive events. Despite the large differences between the samples the results were relatively congruent: 1) long-term mating turned out to be beneficial to fitness, while in contrast, short-term mating was either non-associated or even negatively associated to fitness; 2) long-term mating showed enhanced adaptive benefits for males compared to females; 3) age of first reproduction was the crucial mediating variable in the link between long-term mating and fitness in males; 4) short and long-term mating did not show unambiguous life history dynamics in the context of the fast/slow continuum; however, the obtained findings suggested that long-term mating had more consistent associations with the fast life history dynamics. The data show promising potential in understanding the reproductive ecology of mating in post-industrial humans as well as patterns of sexual selection in contemporary human populations.

Sexual Selection on Mating

Present findings revealed crucial differences in short and long-term mating regarding their relations with fitness: long-term mating showed more positive associations with fitness compared to short-term mating, where no relations or even negative relations with fitness were observed. In Study 1, long-term mating was positively associated with reproductive success and the total desired number of children; it was positively associated both with the number of children and grandchildren in Study 2. In both studies, longer partner relationships were related to an earlier age of first reproduction which turned out to be the crucial mediator between long-term mating and fitness for male participants. The findings that individuals with higher time spent in romantic relationships have higher fitness as well are in accordance not only with the previous findings obtained in post-industrial, WEIRD population (Međedović, 2021) but with the data obtained in rural, natural fertility population - Pimbwe tribe of West Tanzania (Borgerhoff Mulder & Ross, 2019). In contrast, short-term mating was related to delaying reproduction in Study 1 and a lower number of children in Study 2.

We examined Bateman's three coefficients (Arnold & Duvall, 1994; Bateman, 1948) in order to estimate the presence of sexual selection: variance in mating, reproduction, and the association between mating and reproduction. Of course, we should be cautious in the interpretation of variance in mating and fertility: reliable estimations of these parameters should involve representative samples. Our samples were not representative of the Serbian population, although the sample examined in Study 2 had several characteristics of representativeness. Having in mind the problems of results generalizability, it is interesting to mention that all of the effects detected were in congruence with the sexual selection theory: the variation in mating (observed in Study 1 and 2), reproductive success (Study 2) and the finding of higher associations between mating and reproduction in males compared to females (Study 1 and 2). These findings are in accordance with several previous empirical studies (Borgerhoff Mulder & Ross, 2019; Brown et al., 2009; Courtiol et al., 2012; Jokela et al., 2010), although none one of these studies were conducted in industrial and post-industrial human populations. This is particularly interesting since theory and previous data show that sexual selection is weaker in monogamous, compared to polygamous societies (Moorad et al., 2011). Hence, although probably with lower intensity, sexual selection still operates in contemporary humans; more precisely, selection primarily acts to enhance male effort in long-term mating.

Can Mating Patterns Indicate Life History Trajectories?

Apart from sexual selection, mating patterns could be a part of human life history dynamics: correlated traits and events which are associated with fitness. Due to differences in ecological conditions and individual characteristics, humans (like other species as well) may have different pathways of fitness maximization, which are often labeled as fast and slow (Del Giudice et al., 2015). However, there are two opposite hypotheses of the role that mating patterns play in life history dynamics: one assumes that short-term mating represents a part of fast life history trajectory (Belsky et al., 1991; Chisholm, 1999), while the other posits the same role for long-term mating (Gangestad & Simpson, 2000). Both hypotheses have acquired some empirical support but it seems that there are more findings which corroborate the former one (Chua et al., 2016; Copping & Campbell, 2015; Kogan et al., 2015; Lukaszewski, 2015; Schmitt, 2005). The present data did not provide findings which may unambiguously support either of these hypotheses. However, the present data was more in line with strategic pluralism theory (Gangestad & Simpson, 2000). Short term mating showed the signatures of both fast and slow life history while long-term mating exhibited more consistent fast life history dynamics. Indeed, the present data is in the accordance with recent predictions that long-term mating may indicate faster life history dynamics (Sear, 2020): having longer romantic relationships can facilitate reproductive success by higher frequency of sexual intercourse in steady relationships (Twenge et al., 2017) or avoiding the cost of switching partners on reproductive fitness (Brown et al., 2009).

Why were there no clearer associations between mating and life history? Well, the view of life history as a singular slow-fast dimension may be an oversimplifying framework for the analysis of human life histories. Recently, several critiques of the slow-fast life history continuum's existence have been published (Royauté et al., 2018; Stearns & Rodrigues, 2020; Zietsch & Sidari, 2020). Furthermore, empirical data showed that the latent space of life history indicators probably cannot be reduced to a single slow-fast dimension, i.e. it is much more complex and consists of several largely unrelated factors (Međedović, 2020a; 2020b; Richardson et al., 2021). The relations between the parameters of reproductive ecology and childhood environment obtained in the present study (i.e. low magnitude correlations with a high number of non-significant associations) are in contrast to the existence of a singular slow-fast continuum as well. Hence, it is questionable if this simple slow-fast life history theoretical framework is useful for understanding of the mating patterns’ role in life history dynamics. This is why it has been suggested that researchers should invest more effort in linking behavioral traits (like mating patterns) with the specific life history tradeoffs than trying to incorporate them in a rigid and oversimplifying fast-slow continuum (Sear, 2020).

Limitations and Future Directions

There are several important limitations of the present research. As we have already mentioned, the samples of participants the data were collected on were not representative, which limits the generalization of the data (although, the Study 2 was conducted on a large sample which was representative in several demographic parameters). The variation of the reproductive success in Study 1 was diminished which represents a potential obstacle to the generalization of the findings. Additional socio-demographic measures would be useful in the context of present topic - especially the estimate of participants’ income. Participants’ education levels were above the average in the present research; we can reasonably assume that the same holds for their income as well because education and economic status are positively correlated. Hence, the research findings cannot easily generalize to the participants with low education and socioeconomic status. The conducted studies were cross-sectional, which prevents causal inferences from the data; this is a limitation of previous studies in this topic as well. Despite the fact that early fertility is positively associated with completed fertility we should take the measure of reproductive success from Study 1 with caution. We did not use objective information about the participants’ childhood environment but the subjective estimations of ecological characteristics: future research may analyze objective indicators of environment like mortality rates, characteristics of the healthcare system or childhood environmental instability. Furthermore, parental fitness was not controlled for in the present research; future studies should not only control for parental reproductive success but examine the parental influence on mating in offspring, since there is a parent-offspring conflict regarding the mate choice (Buunk et al., 2008).