‘Forever and a Day’ or ‘Just One Night’? On Adaptive Functions of Long-Term and Short-Term Romantic Relationships

On the other hand, parental invest of women in offspring is rather costly (due to a lengthy pregnancy, risks to health during delivery, lactation, and long period of offspring dependency), whereas men have a significantly lower investment in reproduction – ironically, the minimal investment is not even that costly: intercourse. Due to the high cost of a woman’s minimal investment in offspring, women are the choosier sex for whom resources provided by men are crucial. In environments where food resources are scarce and/or there is a high prevalence of predators, surviving of offspring would be endangered in case of an absence of the father's investment. Nowadays a woman is active in the job market, and is able to raise a child independently. However, research finds no correlation between amount of women’s economic participation and their preferences for qualities related to parental care (Gangestad, 1993). Although the availability of birth control significantly reduces the potential costs of short-term mating for women, costs remain higher than for men.

To conclude, short-term mating for women is rather costly as it is associated with risk of raising a child without investment of the father. Short-term mating for men is less risky, or even beneficial as it could provide an opportunity to have offspring at very low cost. Parental uncertainty in humans is a reason why men would be motivated to guard their mate from other men willing to mate with her. As human males invest highly in their offspring compared to most other species, they want to avoid mistakenly investing in another man’s offspring.
Why are not all men engaging only in short-term mating and why are women engaging in short-term mating at all?

Due to the high cost of short-term mating, women are the choosier sex. They tend to make sure that the man is truly willing and has the resources to invest in a long-term relationship. On top of that, women ascertain the potential father to have good parenting skills and good genes to “contribute” to their future offspring. Hence, no matter to what degree men desire short-term mating, finding a partner is not always a possibility. Quite naturally, not every man can attract a high number of short-term mates. For them a beneficial strategy is to invest in one long-term partner and children. Men who are not as attractive as those who engage in short-term mating can trade it off with other qualities – high parental effort, investment and fidelity.

However, women do engage in short-term mating. Research shows that women prefer high social status in long-term mates; conversely, for short-term mates they (like men) prefer physical attractiveness (Li & Kenrick, 2006). Short-term mating for women can be considered as an opportunity to obtain good genes for offspring. Studies comparing behavior of women in fertile (ovulatory) phases to non-fertile phases of their cycle provide good evidence. Women prefer both the scent of symmetrical and masculine male faces more during fertile phases of their menstrual cycle than during their infertile phases (Thornhill & Gangestad, 1999). In another study, women’s preference for men who displayed social presence and direct competitiveness with other men increased on high-fertility days, but only in a short-term mating context (Gangestad, Garver-Apgar & Simpson, 2007). Women who assessed their partners as being relatively lower in sexual attractiveness in high fertility periods than in low fertility periods reported greater desires for extra-pair relationship (Haselton & Gangestad, 2006). Other studies show that self-grooming and ornamentation through choice of attractive outfit increase during the fertile phase of the menstrual cycle (Haselton, Mortezaie, & Pillsworth, 2007). To summarize, engaging in short-term mating with a man of high genetic quality can be beneficial for women. Periodical shifts in the hormonal levels during fertile phases can accordingly produce substantial changes in women’s self presentation, sexual behavior, and preferences in the opposite sex - increasing genetic benefits to an offspring in case of pregnancy.

On top of that, environmental changes have been found to contribute to shifts in trade-offs of mating behavior. If local environment demands parental care of both parents and resources are scarce, investment is preferred over good genes. In other words, women tend to choose a man who is able to provide for their offspring rather than choosing for short-term mating with a man who can provide good genetic fitness. If pathogens prevail in local environments, good genetic fitness can be prefered over investment (Low, 2000).

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