The nature of ancestral human social structure and the circumstances in which men or women tend to be more cooperative are subjects of intense debate. The male warrior hypothesis proposes that success in intergroup contests has been vital in human evolution and that men therefore must engage in maximally effective intragroup cooperation []. Post-conflict affiliation between opponents is further proposed to facilitate future cooperation [], which has been demonstrated in non-human primates [] and humans []. The sex that invests more in post-conflict affiliation, therefore, should cooperate more. Supportive evidence comes from chimpanzees, a close genetic relative to humans that also engages in male intergroup aggression []. Here we apply this principle to humans by testing the hypothesis that among members of a large community, following a conflict, males are predisposed to be more ready than females to repair their relationship via friendly contact. We took high-level sports matches as a proxy for intragroup conflict, because they occur within a large organization and constitute semi-naturalistic, standardized, aggressive, and intense confrontations. Duration or frequency of peaceful physical contacts served as the measure of post-conflict affiliation because they are strongly associated with pro-social intentions []. Across tennis, table tennis, badminton, and boxing, with participants from 44 countries, duration of post-conflict affiliation was longer for males than females. Our results indicate that unrelated human males are more predisposed than females to invest in a behavior, post-conflict affiliation, that is expected to facilitate future intragroup cooperation.

The communicative functions of touch in humans, nonhuman primates, and rats: a review and synthesis of the empirical research.

Results

Sports matches were located following a standardized procedure (see Supplemental Experimental Procedures ). For every match, two coders measured duration of post-conflict affiliation in hundredths of seconds, and the measurements were averaged. Reliability was calculated by correlating coders’ measures, producing r (78) = 0.88 for tennis, r (23) = 0.95 for table tennis, r (23) = 0.93 for badminton, and r (48) = 0.92 for boxing. Because the data were positively skewed and outliers exerted strong effects, we applied a natural log transformation to duration of post-conflict affiliation and then fit the data using two regression models. In the first linear mixed model (LMM), winner’s and loser’s countries were entered as random effects to determine whether these were biasing the effect of sex. A second ordinary least-squares (OLS) regression was then conducted, excluding players’ countries. In all cases, the two models provided similar findings, so we conclude that players’ countries did not influence the results.

Table 1 a a Linear mixed model (LMM) in first column and ordinary least-squares (OLS) in second column. Fixed effects estimates (standard errors) above mid-line; goodness-of-fit statistics by model with random-effects estimates below. Likelihood ratio test (LMM > OLS) in final line. Sex Differences in Post-Conflict Affiliation in Each Sport Tennis b b n = 74 with rank difference information. When rank difference was excluded from models, n = 80, sex (male) estimate = 0.32 for both models, R2 = 0.12, adjusted R2 = 0.07. Badminton Boxing Table Tennis LMM OLS LMM OLS LMM OLS LMM OLS (Intercept) 7.76 7.76 4.43∗∗∗ 4.35∗∗∗ 5.67∗∗∗ 5.61∗∗∗ 3.32∗∗∗ 3.32∗∗∗ (13.58) (13.58) (0.20) (0.18) (0.18) (0.15) (0.16) (0.16) Sex (Male) 0.29∗ 0.29∗ 0.46∗ 0.60∗ 0.95∗∗∗ 0.95∗∗∗ 1.08∗∗∗ 1.08∗∗∗ (0.14) (0.14) (0.21) (0.24) (0.21) (0.21) (0.23) (0.23) Year −0.00 −0.00 – – – – – – (0.01) (0.01) – – – – – – Rank Difference −0.00 −0.00 – – – – – – (0.00) (0.00) – – – – – – Round Number 0.02 0.02 – – – – – – (0.03) (0.03) – – – – – – Total Sets 0.00 0.00 – – – – – – (0.08) (0.08) – – – – – – Akaike Information Criteria 162.49 – 51.73 – 125.27 – 55.43 – Bayesian Information Criteria 183.22 – 57.83 – 134.83 – 61.52 – Log Likelihood −72.24 – −20.87 – −57.63 – −22.71 – Number Observed 74 74 25 25 50 50 25 25 Number of Countries for Losing Players 30 – 10 – 17 – 11 – Number of Countries for Winning Players 28 – 11 – 21 – 12 – Loser’s Country (Intercept) 0.00 – 0.15 – 0.10 – 0.00 – Winner’s Country (Intercept) 0.00 – 0.00 – 0.00 – 0.00 – Var: Residual 0.28 – 0.20 – 0.50 – 0.34 – R2 – 0.09 – 0.22 – 0.29 – 0.48 Adjusted R2 – 0.03 – 0.18 – 0.28 – 0.46 Root-Mean-Square Error – 0.53 – 0.58 – 0.75 – 0.58 Likelihood Ratio Test p Value 1 0.18 1 1 ∗∗∗p < 0.001, ∗∗p < 0.01, ∗p < 0.05. Table 2 Sequential Bonferroni Adjusted p Values for Sex Differences in OLS Models Sport p Value p Value (Adjusted) Tennis a a n = 74. Excluding rank differences from the model, n = 80, p = 0.018, and adjusted p = 0.036. 0.035 0.038 Table Tennis 0.00012 0.00036 Badminton 0.019 0.038 Boxing 0.000048 0.00019 Figure 1 Duration of Post-Conflict Affiliation of Each Match Show full caption Duration of peaceful post-conflict affiliation ordered from shortest to longest within each sport by sex: (A) 40 male and 40 tennis matches, (B) 12 male and 13 female table tennis matches, (C) 15 male and 10 female badminton matches, and (D) 25 male and 25 female boxing matches. For tennis, we located 40 male and 40 female singles tournament matches on YouTube and the Australian Open vault, including 160 unique players from 34 primarily Western countries. To predict duration of post-conflict affiliation, we analyzed sex of players, winner’s and loser’s countries, year of match, difference in players’ rankings, round number in the tournament, and total number of sets played, first with an LMM including players’ countries and then with a simple OLS ( Tables 1 and 2 ). Both models provided similar results (last row of Table 1 ). Only sex was significant, with males engaging in longer durations of post-conflict affiliation than females. Because players’ ranks were unavailable before 1973, six matches were initially not included. Adding these matches into the models by excluding rank difference produced similar results (bottom of Table 1 ). Figure 1 displays the duration of post-conflict affiliation ranked in order from shortest to longest for all matches and shows that males engage in longer durations than females ( Figure 1 A).

Whereas all opponents shook hands following a match, additional touches occurred sporadically. Whether the winner touched the loser with one arm, the loser touched the winner with one arm, or the winner and the loser embraced with two arms or kissed was recorded by each coder (agreement on 75/80 matches for the winner touching the loser, 77/80 matches for the loser touching the winner, and 80/80 matches for the embrace or kiss). A final measure was calculated consisting of whether any physical contact in addition to the handshake occurred between the two players, coded as 0 or 1 (agreement on 78/80 matches). Each coder’s records produced the same results; only one coder’s ratings are included.

Results showed that in significantly more male (17/40 or 42.5%) than female (5/40 or 12.5%) matches, the winner touched the loser’s body with one arm (X2(1) = 9.03, p = 0.003). Further, in significantly more male (9/40 or 22.5%) than female (1/40 or 2.5%) matches, the loser touched the winner’s body with one arm (X2(1) = 7.31, p = 0.007). There was no sex difference in two-armed embraces or kisses (5/40 female matches or 12.5% versus 1/40 male matches or 2.5%), Fisher’s exact test p = 0.20. Overall, in significantly more male matches (21/40 or 52.5%) versus female matches (8/40 or 20%), an additional physical contact occurred (X2(1) = 9.14, p = 0.002).

Choice of matches uploaded to YouTube may introduce bias. In contrast, the International Table Tennis Foundation (ITTF) provides a more systematic sample in its online vault, which contains all table tennis tournament matches from the past 3 years. Every singles finals match containing unique players was included from the vault, producing 12 male and 13 female table tennis matches, with players from 17 countries, half of whom came from China or Japan. To predict post-conflict duration, sex of players and winners’ and losers’ countries were entered into an LMM, and then just sex was included in an OLS. Both models provided similar results, and only sex was significant, with males engaging in longer durations of post-conflict affiliation than females ( Tables 1 and 2 and Figure 1 B).

For table tennis, an additional instance of physical contact besides the handshake occurred in four matches. All were male matches, so that 4/12 or 33% of male matches contained an additional contact, whereas none of the female matches did (X2(1) = 4.80, p = 0.028). (One of the coders also recorded an additional touch in a fifth male match, increasing the size of the effect.)

The Badminton Link vault also provides an unbiased sample of every badminton tournament match from the past 3 years. All finals and semi-finals matches containing unique players were included, yielding 15 male and 10 female matches from 14 primarily East Asian countries. The same analyses were conducted as for table tennis ( Tables 1 and 2 and Figure 1 C). Again, both models provided similar results, and only sex was significant, with males engaging in longer durations of post-conflict affiliation than females. No additional instances of physical contact were visible.

Finally, 25 male and 25 female boxing matches were located on YouTube, including boxers from 29 primarily Western countries, with almost half of the men and the women from the United States. The same analyses were conducted as for badminton ( Tables 1 and 2 and Figure 1 D). Again, both models provided similar results, and only sex was significant, with males engaging in longer durations of post-conflict affiliation than females. Due to the small size of the ring and the large number of individuals interacting with boxers following a match, type of physical contact could not be coded, and only whether boxers’ bodies touched was coded.