Overall, the current study examined how the physical attractiveness and the level of altruism of men affected their desirability to women for both ST and LT relationships. It also builds on the previous research of Barclay (2010) and Farrelly (2011) by using short individual profiles presented to participants and asking them to rate the desirability of each but with the addition of photographs of these individuals portraying them as either high or low in attractiveness. Based on these previous findings and theory outlined earlier, it was predicted that altruists will be rated more desirable than nonaltruists (Hypothesis 1) and that this preference will be greater for LT than ST relationships (Hypothesis 2). Furthermore, as the introduction of physical attractiveness as an additional variable in this study was novel, there is the exploratory hypothesis that this may further affect the findings relating to these first two hypotheses. Finally, based on the previous findings of Fletcher, Tither, O’Loughlin, Friesen, and Overall (2004) , it was predicted that women will show preferences for altruism over physical attractiveness (Hypothesis 3) and that this preference will be greater for LT relationships (Hypothesis 4).

Further support for this is the consistent finding in research of altruism being more desirable in LT partners ( Barclay, 2010 ; Farrelly, 2013 ; Moore et al., 2013 ) as well as altruists being more likely to enter LT relationships ( Stavrova & Ehlebracht, 2015 ) and assortative mating for altruistic characteristics among married couples in Senegal ( Tognetti et al., 2012 ). However, caution needs to be exercised when examining whether a specific trait is a signal of either genetic or phenotypic qualities based on relationship length alone. This is because this may not be a clear indication of what is being signaled, as not all preferences in either ST or LT partners are for traits that signal genetic or phenotypic quality, respectively ( Buss & Shackelford, 2008 ). Similarly, any variation for a complex psychological trait such as altruism will be the product of both genetic and phenotypic/environmental causes. Overall, though, such findings highlight at least in what context women find altruistic men more desirable (which is LT relationships) and as a result contributes to our understanding of altruism’s role in human mate choice.

In total, there were 12 scenarios that participants viewed, 8 of which were altruism conditions and 4 of which were neutral. The order in which participants saw these was randomly determined, and there were two orders that participants were randomly allocated to at the start of the study. As mentioned earlier, each scenario was presented with photographs of two individuals with details of their behavior in relation to the scenario. In each scenario, participants saw a high attractive and a low attractive individual paired together. Across these eight pairings in the “altruism” conditions, high attractive individuals were presented 4 times as being low in altruism and also 4 times as being high in altruism (the same was also true for the low attractive individuals).

Participants first read an information sheet that described the study as investigating female perceptions of attractiveness in different scenarios and then provide informed consent. After giving details of their age, participants were told that they would be required to rate how attractive they found a number of different individuals for both ST and LT relationship and were provided with the descriptions of both relationship types. Participants then went on to view the different men with the accompanying scenarios and behaviors as outlined earlier.

Directly underneath this were two questions that related to each of the individuals. These asked how attractive the person was for two types of relationship, LT or ST. Definitions of these two relationship types (ST: a person with whom you would desire a brief affair or a one night stand. LT: a person with whom you desire a committed LT romantic relationship) were provided based on previous research ( Farrelly, 2011 ). Both questions required a response on a 5-point Likert-type scale from very unattractive to very attractive . All research materials relevant to this article can be accessed by contacting the corresponding author.

Pairs of photographs were presented alongside rubrics of particular scenarios, with the person in each photo given a neutral label (e.g., “Person A”). These scenarios described conditions where individuals could behave altruistically and are based on similar scenarios used elsewhere ( Farthing, 2005 ; Phillips et al., 2008 ). Following this, the rubric described how the two men behaved in response to this scenario, which was either high in altruism or low in altruism. Apart from these, participants also viewed pairs of photographs alongside neutral scenarios, with both individuals described as behaving neither altruistically nor nonaltruistically (e.g. “Person O and Person P both go out clothes shopping, Person O decides to buy a green jumper and Person P buys a pair of jeans”). Examples of the different scenarios are provided in Table 1 .

A series of male facial 2-D photographs were sourced from two different databases, pics.stir.ac.uk and the Radboud Faces Database ( Langner et al., 2010 ), and were then independently assessed by three judges (the three authors) who selected the 12 highest and 12 lowest physically attractive men to be included in the study. Further validation of these selections were from the overall attractiveness ratings in the main study, whereby the high attractive men were rated overall as more attractive than the low attractive men, F (1, 201) = 416.6, p < .001, η 2 = .68.

In terms of whether there was a further influence of the physical attractiveness on the desirability of altruism in LT partners, a significant three-way interaction between attractiveness of target, level of altruism, and relationship length was found, F (1, 201) = 7.27, p = .008, η 2 = .04, see Figure 1 . To better understand this interaction, proportional changes in ratings from ST to LT relationships were calculated and used as the dependent variable in a repeated measures ANOVA with attractiveness of target (high vs. low) and level of altruism (high vs. low) as within-subjects measures. This revealed a significant interaction, F (1, 201) = 5.32, p = .022, η 2 = .03, and subsequent pairwise comparisons revealed that the proportional change from ST to LT was significantly greater for low attractive men than high attractive men only when they displayed high altruism, t (201) = 2.3, p = .022, r = .09, whereas there was no such significant difference when they displayed low altruism, t (201) = −.69, p = .49, r = .05, see Figure 2 . In other words, men low in physical attractiveness were preferred significantly more as LT partners by women only when they were also altruistic.

In terms of whether there was a further influence of the physical attractiveness on the desirability of altruism, a significant interaction was found between target attractiveness and level of altruism, F (1, 201) = 178.28, p < .001, η 2 = .47, see Figure 1 . To further understand the nature of this interaction, the proportional change in the desirability ratings from low attractive to high attractive men was calculated for both low and high altruism levels. A paired t -test then revealed that the proportional increase in ratings from low to high attractive men was greater when they also displayed high altruism than when they also displayed low altruism, t (201) = 8.05, p < .001, r < .27.

Discussion

As predicted, women found men who behaved altruistically more desirable than those who did not. This therefore supports Hypothesis 1 and adds to the body of empirical evidence that shows altruism has an important role in human mate choice. Furthermore, women found altruistic men more desirable for LT relationships, which supports Hypothesis 2 and is in line with previous findings (e.g., Barclay, 2010; Farrelly, 2011, 2013; Oda et al., 2014). As an interestingly aside to this analysis, it was also found that nonaltruistic men were more desirable for ST relationships, which suggests that rather than altruism not being important in ST relationships (as previous research suggests), in the present study, it is actually undesirable. This is a surprising finding, as little is known or researched about what being un-altruistic means in terms of mate choice, and is worthy of further investigation. As such, it may be related to female mate choice for other, less socially desirable traits that may have a role in male ST mating strategies such as the Dark Triad (e.g., Jonason, Li, Webster, & Schmitt, 2009).

The preference for altruistic men was also further influenced by their physical attractiveness, such that the increase in desirability of men as attractiveness increased was significantly greater when they were also altruistic. This suggests that altruism can have an additive effect to other mate choice qualities (in this case, physical attractiveness), such that possessing both qualities has a greater effect on a man’s desirability than just a combination of their individual desirability. Also, the higher preference for LT over ST relationships with men low in attractiveness when they were altruistic suggests that being altruistic may act as a reliable mating strategy for acquiring LT partners for such men. Further investigation, such as examining how men of different attractiveness levels use altruistic acts as LT courtship displays, would explore this possibility in more depth.

Furthermore, the finding that high altruistic/low attractive men were rated more desirable than low altruistic/high attractive men offers support for Hypothesis 3, and further analysis that revealed that this was only significant for LT relationships offers support for Hypothesis 4. This suggests that if a man possesses only one of these traits, it is altruism that is more valuable, particularly for LT relationships. This, together with the above findings, provides further evidence of the importance of altruism in women’s mate choice preferences, even when it is presented with other notable mate choice traits (such as physical attractiveness). Furthermore, by showing that this effect is even greater for LT relationships, more evidence is provided for altruism being a highly important characteristic trait women look for in LT partners.

To properly interpret these findings, it is also important to consider some limitations in the methods used. Firstly, both men in some scenarios (i.e., the one who behaved altruistically and the one that did not) were presented together, such as in the first example in Table 1 (rescuing a child from a river). As a result, this may mean that the nonaltruist in these scenarios may signal other traits as well such as lack of awareness (empathy) or apathy. Although such traits will most likely relate to or be part of a low altruistic nature in such individuals, future research could examine profiles presented alone to avoid any possible confounding effect of comparing the altruism levels of two men simultaneously. Another consideration is that by using a range of different scenarios in this study, possible characteristics may have been signaled other than just altruism. For example, the moral consequences of behaving altruistically in some scenarios were higher than others, for example, comparing attempting to save a child from drowning with buying food for a homeless person. Another example is that altruistic behavior in these two scenarios will also vary in the degree to which physical strength/prowess is also signaled, which will be higher when jumping into a river to save a child than when buying a sandwich. Overall, though, all scenarios used in this study can be considered to act as reliable signals of altruism, even if individual scenarios may be able to signal other traits as well. This therefore makes the findings in this study from women’s ratings of altruistic behavior across all scenarios still valid. Future research however may wish to explore the different types of altruism signaled in these individual scenarios separately in more detail, such as heroism (Farthing, 2005, 2007; Kelly & Dunbar, 2001) or charitable giving (e.g., Barclay, 2010; Iredale et al., 2008). Such research would need to control for the extraneous variables that may also be signaled (e.g., physical strength across different heroic scenarios) to ensure that only the value of that type of altruism in mate choice is reliably being examined.

It will be important for additional research to build on these findings in certain directions to further aid our understanding. Firstly, an examination of menstrual cycle effects may shed some light on the results obtained here. In particular, how they may affect the potential weightings of altruism and physical attractiveness in women’s ratings of men’s desirability across the cycle. Also, future research needs to examine the effects found here in men’s ratings of the desirability of altruistic women as well. This is because there is a lack of such research in this area, as most studies have concentrated on only women’s ratings. However, when both sexes are investigated, it has led to similar findings (e.g., Farrelly, 2013; Farrelly et al., 2007; Moore et al., 2013; Stavrova & Ehlebracht, 2015), suggesting that altruism has been shaped by mutual mate choice in humans (Miller, 2000). It is therefore important to see whether this is also the case when physical attractiveness is also explored alongside it. Also of value would be an exploration of the interaction of the different variables used here in real-world mate choice settings. Experimentally derived findings on mate choice decision making such as those outlined here can only benefit from seeing if the same findings are present when looking at actual relationships and interactions in the real world, which is a methodology successfully employed elsewhere (e.g., Phillips et al., 2008; Stavrova & Ehlebracht, 2015).

To conclude, these findings make a substantial contribution to our understanding of the role of altruistic behavior in human mate choice by including the previously unexplored additional variables of physical attractiveness. Overall, the results provide further support for the view that altruism acts as an important trait in mate choice, particularly for LT relationships. This has important implications for our understanding of how we understand the expression of altruism and other traits holistically by casting further light on the rich tapestry that is human mating. This is also evident in the fact that the current study also suggests that nonaltruism may have a role in mate choice, perhaps aligning the spectrum of behavior explored here with research on traits such as the Dark Triad. As such, the current study can aid our understanding of altruism not only in mate choice but also in different everyday occurrences of this ubiquitous but enigmatic trait.