These differences in skeletal structure and the associated throwing competencies, combined with the large male advantage in arm and upper body strength, indicate strong selection pressures for these physical competencies in men. In fact, these sex differences are consistent with the view that the evolution of male-male competition in humans was influenced by the use of projectile (e.g., spears) and blunt force (e.g., clubs) weapons (Keeley, 1996; see also the Physical modules section of Chapter 8); during agonistic encounters, male chimpanzees often use projectile weapons (e.g., stones) and sticks as clubs and do so much more frequently than female chimpanzees (Goodall, 1986). The finding that men have a higher threshold and greater tolerance for physical pain than do women, on average, is also in keeping with the view that male-male competition is related to human physical dimorphisms, given that success at such competition is almost certainly facilitated by the ability to endure physical pain (Berkley, 1997; Velle, 1987); of course, women can endure considerable pain under some circumstances, such as childbirth.
Nonetheless, it might be argued that these physical sex differences have emerged from a sex difference in the division of labor, such as hunting, rather than direct male-male competition (e.g., Frost, in press; Kolakowski & Malina, 1974). Although the sexual division of labor contributes to the differential mortality of men and women in preindustrial societies and might influence the reproductive variance of men, comparative studies of the relation between physical dimorphisms and male-male competition suggest that the sexual division of labor is not likely to be the primary cause of these physical dimorphisms (see also The evolution of sex differences and the sexual division of labor section of Chapter 3). Recall, for primates and many other species, there is a consistent relation between physical sex differences and the nature of intrasexual competition (see Chapter 3). For monogamous primates—those with little direct male-male competition over access to mating partners—there are little or no differences in the physical size or the pattern of physical development of females and males (Leigh, 1995). For nonmonogamous primates—those characterized by direct male-male competition over access to mating partners—males are consistently larger than females, and this difference in physical size is consistently related, across species, to the intensity of physical male-male competition and not to the foraging strategy of the species (Clutton-Brock et al., 1977; Mitani et al., 1996; Plavcan & van Schaik, 1997a).
...
On the basis of the sex differences in parental investment (Chapter 4), the nature of intrasexual competition and in mate choice criteria (Chapter 5) in adulthood, sex differences in the self-initiated developmental experiences of boys are girls are expected and are found. Although there are, of course, many similarities in the childhood experiences of boys and girls, there are also considerable differences. Girls and boys show different patterns of physical development (Tanner, 1990), different play interests and styles, as well as different social behaviors and motives, and many of these differences can be readily understood in terms of sexual selection in general and intrasexual competition in particular (Darwin, 1871).
As an example, the delayed physical maturation of boys, relative to girls, and the sex difference in the timing, duration, and intensity of the pubertal growth spurt follow the same pattern as is found in other nonmonogamous primates (Leigh, 1995, 1996). Across these primate species, the sex differences in these features of physical maturation are consistently related to the intensity of physical male-male competition, as contrasted with any sex differences in foraging strategy (Mitani et al., 1996). The sex difference in the pattern of human physical competencies, such as a longer forearm and greater upper body strength in men than in women, is also readily explained in terms of selection for male-on-male aggression, selection that involved the use of projectile and blunt force weapons (Keeley, 1996). Stated more directly, the sex differences in physical development and physical competencies have almost certainly been shaped by sexual selection, and the majority of these differences have resulted from male-male competition over access to mates (Tanner, 1992); of course, some physical sex differences, such as the wider pelvis in women, have been shaped by natural selection.
It is very likely that many of the sex differences in play interests and social behaviors have also been shaped by sexual selection. The sex differences in rough-and-tumble play, exploratory behavior and size of the play range, the tendency of boys to form coalitions in their competitive activities with other boys, and the formation of within-coalition dominance hierarchies are also patterns that are associated with male-male competition in other primates, particularly primates in which males are the philopatric sex (Goodall, 1986; Smith, 1982). In this view, all of these features of boys’ play and social behavior involve a preparation for later within-group dominance striving and coalition formation for intergroup aggression. Through parenting practices, such as degree of physical discipline, the selective imitation of competitive activities, and actual experiences within same-sex groups, boys learn how to best achieve within-group social dominance and practice the specific competencies associated with male-male competition in their particular culture. They learn how to achieve cultural success (e.g., by leading raids on other villages or becoming a star football player).
Not all developmental sex differences are related to male-male competition, however. For instance, the relational aggression that is common in girls’ groups might be a feature of female-female competition and a number of other physical and behavioral sex differences that become evident during development have likely been shaped by natural selection or mate choice. The sex difference, favoring boys, in manipulative and exploratory object-oriented play appears to be related to the evolution of tool use and a sex difference, favoring men, in the range of tool-related activities in adulthood. Although it is not certain, these sex differences have likely been shaped, in part, by natural selection (e.g., through a sex difference in the foraging strategies of our ancestors). Similarly, the sex difference in play parenting, favoring girls, reflects the later sex difference in parental investment, favoring women, and has almost certainly been shaped by natural selection (Pryce, 1995). Finally, many of the physical changes associated with puberty, such as the development of a masculine jaw in men and relatively large breasts in women, have likely been shaped by the mate choice preferences of the opposite sex and might be condition-dependent indictors of physical and genetic health (Thornhill & Müller, 1997).
Thanks for tracking that down. I’m more sympathetic to this point of view than I was, but I think that one of its premises is unfounded. From your selection:
The sex difference in the pattern of human physical competencies, such as a longer forearm and greater upper body strength in men than in women, is also readily explained in terms of selection for male-on-male aggression, selection that involved the use of projectile and blunt force weapons (Keeley, 1996).
That citation may answer my doubt, but this argument seems to be undermined by the extensive use of weapons early homo sapiens made while hunting, a point where they diverged pretty sharply from other primates. Weapon-use was also involved in male-male aggression, yes, but it doesn’t seem overwhelmingly clear that sexual selection was the primary factor.
That citation may answer my doubt, but this argument seems to be undermined by the extensive use of weapons early homo sapiens made while hunting, a point where they diverged pretty sharply from other primates.
Other primates used weapons, too. Here is a famous one. Was the target of the first man-thrown projectile an animal while hunting, or another man? Who knows, but once one use was discovered, the second probably followed soon after.
Weapon-use was also involved in male-male aggression, yes, but it doesn’t seem overwhelmingly clear that sexual selection was the primary factor.
Sexual selection has two components: intersexual selection (mate choice based on preferences of the opposite sex) and intrasexual competition/selection (competition within members of your own sex. If weapon-use was related to male-male aggression, then it was related to the intrasexual competition component of sexual selection.
Clearly, weapon-use would be beneficial both for hunting and warfare. But I wonder if these selection pressures were stronger in warfare? Animals don’t throw spears back at you.
Furthermore, even if male weapon-use is adapted for hunting, that doesn’t necessary mean we are only seeing pressures from natural selection, not sexual selection also. As far as I remember from one of my anthropology classes, most of hunter-gatherer calories in certain societies don’t come from hunting. The hypothesis is that male hunting skills aren’t emphasized because of their importance for feeding people, which could be acted on by natural selection; rather, hunting had become yet another domain where males competed with each other to gain status and attract females. I can try to find a source on this.
Right. I don’t disagree that male-male aggression played some role. I am just uncertain about whether it exerted a stronger selection pressure than the natural selection effect of hunting ability on survival.
The sudden adoption of weapons which required upper body strength doesn’t provide much evidence to distinguish between the two.
And don’t feel obligated to go out of your way to track down sources for me; I’d prefer to know more about this issue, ceteris paribus, but I don’t think it’s terribly important in the grand scheme of things.
Some of Male, Female is online, so you can see Geary’s reasoning (emphases mine):
These differences in skeletal structure and the associated throwing competencies, combined with the large male advantage in arm and upper body strength, indicate strong selection pressures for these physical competencies in men. In fact, these sex differences are consistent with the view that the evolution of male-male competition in humans was influenced by the use of projectile (e.g., spears) and blunt force (e.g., clubs) weapons (Keeley, 1996; see also the Physical modules section of Chapter 8); during agonistic encounters, male chimpanzees often use projectile weapons (e.g., stones) and sticks as clubs and do so much more frequently than female chimpanzees (Goodall, 1986). The finding that men have a higher threshold and greater tolerance for physical pain than do women, on average, is also in keeping with the view that male-male competition is related to human physical dimorphisms, given that success at such competition is almost certainly facilitated by the ability to endure physical pain (Berkley, 1997; Velle, 1987); of course, women can endure considerable pain under some circumstances, such as childbirth.
Nonetheless, it might be argued that these physical sex differences have emerged from a sex difference in the division of labor, such as hunting, rather than direct male-male competition (e.g., Frost, in press; Kolakowski & Malina, 1974). Although the sexual division of labor contributes to the differential mortality of men and women in preindustrial societies and might influence the reproductive variance of men, comparative studies of the relation between physical dimorphisms and male-male competition suggest that the sexual division of labor is not likely to be the primary cause of these physical dimorphisms (see also The evolution of sex differences and the sexual division of labor section of Chapter 3). Recall, for primates and many other species, there is a consistent relation between physical sex differences and the nature of intrasexual competition (see Chapter 3). For monogamous primates—those with little direct male-male competition over access to mating partners—there are little or no differences in the physical size or the pattern of physical development of females and males (Leigh, 1995). For nonmonogamous primates—those characterized by direct male-male competition over access to mating partners—males are consistently larger than females, and this difference in physical size is consistently related, across species, to the intensity of physical male-male competition and not to the foraging strategy of the species (Clutton-Brock et al., 1977; Mitani et al., 1996; Plavcan & van Schaik, 1997a).
...
On the basis of the sex differences in parental investment (Chapter 4), the nature of intrasexual competition and in mate choice criteria (Chapter 5) in adulthood, sex differences in the self-initiated developmental experiences of boys are girls are expected and are found. Although there are, of course, many similarities in the childhood experiences of boys and girls, there are also considerable differences. Girls and boys show different patterns of physical development (Tanner, 1990), different play interests and styles, as well as different social behaviors and motives, and many of these differences can be readily understood in terms of sexual selection in general and intrasexual competition in particular (Darwin, 1871).
As an example, the delayed physical maturation of boys, relative to girls, and the sex difference in the timing, duration, and intensity of the pubertal growth spurt follow the same pattern as is found in other nonmonogamous primates (Leigh, 1995, 1996). Across these primate species, the sex differences in these features of physical maturation are consistently related to the intensity of physical male-male competition, as contrasted with any sex differences in foraging strategy (Mitani et al., 1996). The sex difference in the pattern of human physical competencies, such as a longer forearm and greater upper body strength in men than in women, is also readily explained in terms of selection for male-on-male aggression, selection that involved the use of projectile and blunt force weapons (Keeley, 1996). Stated more directly, the sex differences in physical development and physical competencies have almost certainly been shaped by sexual selection, and the majority of these differences have resulted from male-male competition over access to mates (Tanner, 1992); of course, some physical sex differences, such as the wider pelvis in women, have been shaped by natural selection.
It is very likely that many of the sex differences in play interests and social behaviors have also been shaped by sexual selection. The sex differences in rough-and-tumble play, exploratory behavior and size of the play range, the tendency of boys to form coalitions in their competitive activities with other boys, and the formation of within-coalition dominance hierarchies are also patterns that are associated with male-male competition in other primates, particularly primates in which males are the philopatric sex (Goodall, 1986; Smith, 1982). In this view, all of these features of boys’ play and social behavior involve a preparation for later within-group dominance striving and coalition formation for intergroup aggression. Through parenting practices, such as degree of physical discipline, the selective imitation of competitive activities, and actual experiences within same-sex groups, boys learn how to best achieve within-group social dominance and practice the specific competencies associated with male-male competition in their particular culture. They learn how to achieve cultural success (e.g., by leading raids on other villages or becoming a star football player).
Not all developmental sex differences are related to male-male competition, however. For instance, the relational aggression that is common in girls’ groups might be a feature of female-female competition and a number of other physical and behavioral sex differences that become evident during development have likely been shaped by natural selection or mate choice. The sex difference, favoring boys, in manipulative and exploratory object-oriented play appears to be related to the evolution of tool use and a sex difference, favoring men, in the range of tool-related activities in adulthood. Although it is not certain, these sex differences have likely been shaped, in part, by natural selection (e.g., through a sex difference in the foraging strategies of our ancestors). Similarly, the sex difference in play parenting, favoring girls, reflects the later sex difference in parental investment, favoring women, and has almost certainly been shaped by natural selection (Pryce, 1995). Finally, many of the physical changes associated with puberty, such as the development of a masculine jaw in men and relatively large breasts in women, have likely been shaped by the mate choice preferences of the opposite sex and might be condition-dependent indictors of physical and genetic health (Thornhill & Müller, 1997).
Thanks for tracking that down. I’m more sympathetic to this point of view than I was, but I think that one of its premises is unfounded. From your selection:
That citation may answer my doubt, but this argument seems to be undermined by the extensive use of weapons early homo sapiens made while hunting, a point where they diverged pretty sharply from other primates. Weapon-use was also involved in male-male aggression, yes, but it doesn’t seem overwhelmingly clear that sexual selection was the primary factor.
Other primates used weapons, too. Here is a famous one. Was the target of the first man-thrown projectile an animal while hunting, or another man? Who knows, but once one use was discovered, the second probably followed soon after.
Sexual selection has two components: intersexual selection (mate choice based on preferences of the opposite sex) and intrasexual competition/selection (competition within members of your own sex. If weapon-use was related to male-male aggression, then it was related to the intrasexual competition component of sexual selection.
Clearly, weapon-use would be beneficial both for hunting and warfare. But I wonder if these selection pressures were stronger in warfare? Animals don’t throw spears back at you.
Furthermore, even if male weapon-use is adapted for hunting, that doesn’t necessary mean we are only seeing pressures from natural selection, not sexual selection also. As far as I remember from one of my anthropology classes, most of hunter-gatherer calories in certain societies don’t come from hunting. The hypothesis is that male hunting skills aren’t emphasized because of their importance for feeding people, which could be acted on by natural selection; rather, hunting had become yet another domain where males competed with each other to gain status and attract females. I can try to find a source on this.
Right. I don’t disagree that male-male aggression played some role. I am just uncertain about whether it exerted a stronger selection pressure than the natural selection effect of hunting ability on survival.
The sudden adoption of weapons which required upper body strength doesn’t provide much evidence to distinguish between the two.
And don’t feel obligated to go out of your way to track down sources for me; I’d prefer to know more about this issue, ceteris paribus, but I don’t think it’s terribly important in the grand scheme of things.