Sexual selection, especially competition among males for access to mates, is the primary evolutionary force behind the strength gap between men and women. In species where males physically contest each other for reproductive opportunities, bigger muscles, heavier bones, and more powerful limbs give a male a direct reproductive edge. Across carnivores, primates, and humans, the pattern holds: the more intense the male-male competition in a species’ mating system, the more pronounced the physical differences between the sexes. But the story is richer than “males fight, so males got strong,” and in many lineages the pattern flips entirely.
How Large Is the Gap, and Where Does It Show Up Most
The strength difference between men and women is real and well measured, but it is not uniform across the body. Research stretching back decades consistently finds that women’s upper-body strength is roughly 50 to 60 percent of men’s, while lower-body strength sits around 60 to 70 percent of men’s, and trunk strength about 60 percent.1The Journal of Strength & Conditioning Research. Narrative Review of Sex Differences in Muscle Strength, Endurance, Activation, Size, Fiber Type, and Strength Training Participation Rates, Preferences, Motivations, Injuries, and Neuromuscular Adaptations A study of university students found that women’s strength corresponded to just over half of men’s, closely mirroring their lean-mass ratio.2PubMed Central. Sex differences in upper and lower strength and their association with body composition among university students The lopsided upper-versus-lower-body pattern is one of the most telling clues about why the gap exists in the first place.
The gap also does not appear overnight at puberty. A large meta-analysis tracking children and adolescents found that boys between ages five and ten already had about 17 percent greater upper-limb strength and 8 percent greater lower-limb strength than girls of the same age. By ages 14 to 17, male puberty had dramatically widened those numbers to roughly 50 percent greater upper-limb strength and 30 percent greater lower-limb strength.3PubMed Central. Sex Differences in Upper- and Lower-Limb Muscle Strength in Children and Adolescents: A Meta-Analysis The pre-pubertal difference is small but detectable, meaning some of the gap is baked in before testosterone surges during adolescence. Puberty then acts as an amplifier, not the sole creator.
Sexual Selection and the Competition for Mates
The core evolutionary explanation is straightforward: in many species, a male’s lifetime reproductive success depends on his ability to outcompete other males physically. This does not have to mean fighting to the death. It can mean shoving rivals away from females, winning displays of strength, or simply being intimidating enough that competitors back off. Over many generations, males who were bigger and stronger left more offspring, and those offspring inherited the traits that made their fathers successful.
A large comparative study of carnivores tested this idea directly by measuring skeletal shape differences between males and females across many species. Males consistently showed skeletal traits associated with greater aggressive performance, and the degree of that dimorphism tracked tightly with mating system: species where one male monopolizes multiple females (polygyny) showed more pronounced skeletal differences between the sexes than species with less intense male-male competition.4Evolution. Sexual selection on skeletal shape in Carnivora Skeletal shape dimorphism also correlated with body size dimorphism, a standard proxy for the intensity of sexual selection, reinforcing that these traits are driven by reproductive competition rather than ecological factors like diet.
In primates, the picture is similar. Comparative analyses have confirmed that sexual selection plays a major role in primate dimorphism, with mating systems strongly predicting how different males and females look. Researchers have also found that dimorphism results from changes in both male and female traits, not just males getting bigger.5PubMed Central. Sexual dimorphism in primate evolution That last point is subtle but important: natural selection can simultaneously push female body size down (or hold it steady) while sexual selection pushes male size up. The gap is a product of forces acting on both sexes, not just one.
The Upper-Body Puzzle and the Punching Hypothesis
If the sex difference in strength were purely about overall body size, you would expect a roughly even gap everywhere. Instead, the upper body, especially the arms and shoulders, shows a disproportionately large difference. Researchers have proposed that this reflects selection specifically on male fighting ability. A study measuring arm cranking power found greater sexual dimorphism in forward arm power (the motion used in striking) than in backward arm power. Males also showed a higher ratio of forward-to-backward power, consistent with selection having shaped the muscles used in punching and striking, not just general upper-body tasks.6Journal of Experimental Biology. Sexual dimorphism in human arm power and force: implications for sexual selection on fighting ability
This does not mean all human male ancestors were brawlers. The hypothesis is about statistical tendencies over deep evolutionary time: males who could strike harder won more confrontations, whether those confrontations were common or rare. Over tens of thousands of generations, even a modest advantage in fighting ability can produce a substantial shift in musculature if it consistently translates into reproductive success.
What Testosterone Does to Muscle
The hormonal mechanism behind the strength gap centers on androgens, particularly testosterone acting through the androgen receptor. Research using mice genetically engineered to lack the androgen receptor showed that males without it had decreased muscle mass and reduced force production from fast-twitch muscle. Females missing the same receptor had entirely normal muscle mass. In other words, androgens are not required for female muscle development but are critical for building and maintaining the larger male musculature.7PubMed. Impaired skeletal muscle development and function in male, but not female, genomic androgen receptor knockout mice
At the molecular level, androgens appear to promote muscle growth by keeping muscle precursor cells in a proliferating state, delaying their maturation into final muscle fibers and allowing more material to accumulate. They also regulate enzymes involved in polyamine production, which is part of the cellular machinery that supports tissue growth. The effect is not a single switch but a network of gene pathways that collectively give male muscle its larger baseline size and greater force capacity.
Testosterone also comes with trade-offs. It is an immunosuppressant, and males across vertebrate species show evidence of a tension between investing energy in reproduction and immune defense. High testosterone supports muscle mass and competitive ability, but it appears to come at the cost of some immune function.8American Journal of Human Biology. Testosterone-mediated immune functions and male life histories This is a classic life-history trade-off: energy spent on being big and strong is energy not spent on staying healthy.
Fiber Types, Force Production, and the Molecular Details
Not all muscle fibers are created equal, and men and women differ in fiber composition. A meta-analysis pooling data from living men and women found that men have larger cross-sectional areas for all fiber types, a higher proportion of fast-twitch Type II fibers, and greater Type II-to-Type I fiber area ratios. Women, by contrast, have a higher proportion of slow-twitch Type I fibers.9PubMed. Sex differences in skeletal muscle fiber types: A meta-analysis Fast-twitch fibers generate more force in quick bursts. Slow-twitch fibers are better at sustained, lower-intensity work. This fiber-type split helps explain why men tend to produce more peak force while women often show greater fatigue resistance in certain tasks.
The force difference goes beyond fiber quantity. When researchers measured force production from individual muscle fibers, they found that the fastest fiber type (type IIx) generated about 60 percent more force per unit of cross-sectional area in men than in women. Other fiber types showed no significant sex difference in force per unit area.10Journal of Exercise Rehabilitation. Sex- and fiber-type-related contractile properties in human single muscle fiber So it is not just that men have more muscle; the fibers themselves, at least the fastest ones, are mechanically different.
At the gene-expression level, the picture gets more interesting. A study of human skeletal muscle found that women had more than double the expression of two genes that act as brakes on muscle growth: GRB10, which inhibits a key growth-factor signaling pathway, and ACVR2B, the receptor for myostatin, a protein that limits muscle size.11PubMed Central. Sex-related differences in gene expression in human skeletal muscle Higher expression of these growth-limiting genes in women may help explain why female muscle stays smaller even when exposed to resistance training. Meanwhile, men showed higher expression of genes encoding mitochondrial and ribosomal proteins, consistent with greater capacity for energy production and protein synthesis in muscle.
After a bout of resistance exercise, sex differences in gene regulation persist. One study found that males experienced prolonged changes in muscle gene expression while females showed a rapid return to baseline, with some signaling pathways activated only in one sex or the other.12BMC Genomics. Skeletal muscle gene expression in response to resistance exercise: sex specific regulation Intriguingly, the cellular response to training at the level of muscle stem cells (satellite cells) appears similar between men and women, suggesting that the basic machinery for building new muscle is shared, even if the hormonal and genetic context produces different outcomes.13PubMed. Satellite cell and myonuclear accretion is related to training-induced skeletal muscle fiber hypertrophy in young males and females
Bones Tell the Same Story
The strength gap is not only about muscle. A study matching 18-year-old males and females for identical height and weight found that males still had greater bone mineral content and density at the hip and distal tibia, along with thicker cortical bone.14PubMed. Males have larger skeletal size and bone mass than females, despite comparable body size Geometric measures of the hip, including the width of the femoral neck and overall structural safety factors, were all higher in males. These differences mean that even at the same body size, the male skeleton is built to withstand more mechanical stress. In an evolutionary context, this is consistent with selection for physical robustness in males: a skeleton optimized for absorbing and delivering impact loads during aggression or competition.
Early hominid fossils tell us that sex differences in body size were substantially larger in our distant ancestors. Species like Australopithecus appear to have been more dimorphic than modern humans, with the gap narrowing as the genus Homo evolved.15PubMed Central. Equality for the sexes in human evolution? Early hominid sexual dimorphism and implications for mating systems and social behavior This reduction in dimorphism over evolutionary time is often interpreted as evidence that human mating systems shifted away from intense male-male physical competition as pair-bonding, tool use, and cooperation became more important to survival.
Neuromuscular Wiring Differs Too
Even the nervous system’s control of muscle shows sex-specific patterns, which adds another layer to the strength difference. Research using high-density surface electromyography found that women’s lower-threshold motor units (the ones recruited for lighter tasks) fire at higher rates, while men’s higher-threshold motor units (recruited for heavy, powerful efforts) fire at higher rates.16PubMed Central. Characterising sex-related differences in lower- and higher-threshold motor unit behaviour through high-density surface electromyography Recruitment thresholds are also higher in males across all force levels, meaning their nervous systems bring larger motor units online at higher demand levels.
Women also show evidence of greater self-sustained firing in motor neurons, which functionally allows their muscles to keep contracting with less ongoing neural input. Research tracking motor unit behavior across development found that females of all ages had larger self-sustained firing values than males.17PubMed Central. Sex-related differences in motoneuron firing behavior during typical development Additionally, when men and women are matched for strength, women show even higher motor unit discharge rates and more frequent rapid discharges, suggesting they need to generate greater neural drive to achieve the same force output from smaller muscles.18PubMed. Sex differences in motor unit discharge rates at maximal and submaximal levels of force output The female neuromuscular system, in other words, is not simply a weaker version of the male one. It is wired differently in ways that trade peak force for sustained activity and fatigue resistance.
Female Advantages in Ultra-Endurance
The emphasis on peak strength can obscure the fact that women have physiological traits that would be expected to provide advantages in other physical domains. A review of ultra-endurance performance found that women exhibit greater fatigue resistance, more efficient use of fat as fuel, and lower overall energy demands per unit of body mass during prolonged activity.19PubMed. Do Sex Differences in Physiology Confer a Female Advantage in Ultra-Endurance Sport? These traits, rooted in the same hormonal and fiber-type differences that reduce peak strength, could represent their own form of evolutionary optimization. A body designed to sustain activity over long periods while carrying or nursing offspring faces very different selection pressures than one designed to win brief, violent contests.
Pregnancy itself imposes significant physical costs. Carrying a front-loaded mass of up to 16 percent of body weight slows women’s optimal walking speed and increases the energy cost of locomotion.20Journal of Human Evolution. Reproductive costs for everyone: How female loads impact human mobility strategies Over evolutionary time, selection on the female body has had to optimize for carrying and delivering offspring, nursing, and sustaining physical activity for months while pregnant. Investing heavily in upper-body muscle mass used for fighting would not only be irrelevant to these demands, it would be metabolically expensive to maintain.
When Females Are the Bigger, Stronger Sex
The male-larger pattern is not universal, and the exceptions are revealing. In many species, particularly among invertebrates, fish, and reptiles, females are the larger sex. This is called female-biased sexual size dimorphism, and it is widespread enough to be considered the norm in several animal groups.
In crab spiders, females can be dramatically larger than males. One study found that in species using lucrative foraging strategies like flower-ambush hunting, females had a body width 91 percent larger than males, compared with 26 percent larger in species with less profitable foraging tactics.21PubMed Central. Foraging strategy as a route for sexual size dimorphism evolution The likely explanation is fecundity selection: larger females produce more eggs, and when food is abundant enough to support extreme female growth, natural selection favors larger and larger females. Meanwhile, males may actually be selected to stay small, because smaller males can move faster to find mates in a “scramble competition” system where speed matters more than fighting.
Among geckos, miniaturized species tend to show more female-biased dimorphism. In the family containing many of these tiny species, larger species were male-biased in size while smaller species were more female-biased, consistent with the idea that very small females face strong selection to remain large enough to produce viable eggs.22PubMed. The effect of miniaturization on the evolution of sexual size dimorphism in geckos There is a floor below which a female simply cannot shrink without compromising reproduction, and that floor creates a pattern where the smallest gecko species end up female-biased.
The spotted hyena is perhaps the most dramatic mammalian example of a reversal. Female spotted hyenas dominate males socially and physically. Researchers have proposed that this evolved because young hyenas take an unusually long time to develop the skull and jaw strength needed to crack bones, their primary food source. Between weaning and full skull maturity, juvenile survival depends heavily on their mother’s ability to aggressively secure food access. Mothers who could displace competitors from kills kept more offspring alive, driving selection for increased female size and aggression.23PubMed Central. Post-weaning maternal effects and the evolution of female dominance in the spotted hyena In hyenas, the selection pressure that usually favors bigger males was overwhelmed by a different pressure favoring bigger, more aggressive mothers.
Size Differences That Reduce Competition for Food
Not every explanation for size dimorphism is about fighting or fecundity. An alternative hypothesis proposes that when males and females differ in size, they can exploit different food resources, reducing competition between the sexes within the same population. This “niche partitioning” idea has support in several species. In North American dabbling ducks, the size difference between males and females is consistent with the hypothesis that intersexual competition for food drives niche divergence and contributes to size dimorphism.24Canadian Journal of Zoology. Sexual size dimorphism in relation to resource partitioning in North American dabbling ducks
A similar finding emerged from fishers (a medium-sized carnivore in North America), where differences in body and skull size between males and females provide partial support for the idea that dimorphism evolved to lessen intraspecific competition.25Canadian Journal of Zoology. Linking sexual size dimorphism to trophic niche partitioning in a generalist predator In practice, sexual selection and niche partitioning are not mutually exclusive. Both can operate simultaneously, with fighting ability driving the initial divergence in size and ecological separation reinforcing it once males and females are different enough to exploit different prey or food patches.
For humans, niche partitioning is probably a minor player compared to sexual selection. But the existence of the pattern in other species reminds us that “males are bigger because they fight” is not the only evolutionary pathway to dimorphism. It is the dominant one in primates and many mammals, but evolution is opportunistic, and the same physical outcome can arise from different selective pressures depending on a species’ ecology, mating system, and life history.