What makes a man a man depends on who’s asking and what lens they’re looking through. Biology offers one answer: a cascade of genetic and hormonal events that begin weeks after conception. Psychology offers another: an internal sense of identity that takes shape in the brain before birth. And culture adds yet another layer, with societies around the world defining manhood through rituals, roles, and expectations that vary enormously from place to place. No single factor works alone. Being male is the product of genetics, hormones, brain development, and lived experience all interacting across a lifetime.
It Starts With a Single Gene
The biological story of maleness begins with a gene called SRY, located on the Y chromosome. Around six weeks after conception, this gene activates in an embryo that has inherited a Y chromosome from its father. SRY produces a protein that binds to DNA and dramatically distorts its shape, which in turn switches on a series of other genes. The result is the formation of testes from tissue that would otherwise develop into ovaries.
Once the testes form, they begin producing testosterone, and this single hormone drives most of what we think of as male biological development. The genitals differentiate during the first trimester of pregnancy. The brain begins its own hormone-influenced development in the second trimester. Between months two and six of pregnancy, and again from one to three months after birth, testosterone surges shape the developing brain in ways that appear to be permanent.
Biology doesn’t always follow the textbook path, though. About 1 in 20,000 people who appear male have two X chromosomes instead of an X and a Y. This happens when the SRY gene accidentally gets transferred onto an X chromosome during the formation of the father’s sperm. The resulting embryo develops male anatomy despite having no Y chromosome. Cases like these show that maleness isn’t simply “having a Y chromosome.” It’s about which genetic instructions get activated and when.
What Testosterone Does to the Body
Puberty is where the biological blueprint becomes visible. A surge in testosterone, combined with growth hormone and a growth factor called IGF-1, triggers the male growth spurt. Bones lengthen and increase in density. Muscles grow. The circulatory and respiratory systems expand rapidly, giving adolescent boys increasing physical capacity. The voice deepens as the larynx enlarges. Facial and body hair appear. The shoulders broaden relative to the hips.
In healthy adult men between 19 and 39, testosterone levels typically fall between 264 and 916 nanograms per deciliter, according to a landmark study by the Endocrine Society that established harmonized reference ranges. That’s a wide range, which means two perfectly healthy men can have testosterone levels that differ by a factor of three. The hormone continues to influence muscle maintenance, fat distribution, bone density, red blood cell production, and sex drive throughout life.
How the Male Brain Develops Differently
The brain is sexually dimorphic, meaning it develops somewhat differently in males and females, though with enormous overlap between individuals. In childhood and adolescence, males have on average about 9% larger total brain volume. The amygdala, a region involved in processing emotions like fear and aggression, grows significantly more in males during development. The hippocampus, which plays a role in memory, grows more in females. The globus pallidus, involved in voluntary movement, is proportionally larger in males.
These differences emerge from the testosterone exposure that begins in the womb. During a critical window in prenatal development, testosterone and its byproducts influence which neurons survive, how they connect to each other, and what chemical signals they use. Researchers describe these as “organizing” effects because they set up the brain’s architecture permanently, in contrast to the “activating” effects of hormones after puberty, which are temporary and fluctuate with hormone levels. The organized differences appear to contribute to statistical patterns in behavior, cognition, and identity, though no brain scan can reliably identify whether an individual is male or female.
Gender Identity: The Psychological Dimension
Gender identity is a person’s internal sense of being male, female, or something else entirely. The American Psychological Association describes it as a deeply felt, inherent sense that may or may not correspond to a person’s physical sex characteristics. For the vast majority of people, gender identity aligns with their biology. For a small percentage, it doesn’t.
The neuroscience suggests that gender identity has biological roots. The brain’s sexual differentiation happens on a separate timeline from genital development. Because the genitals form in the first trimester and the brain differentiates starting in the second trimester, these processes can theoretically be influenced by different hormonal environments. This is one proposed explanation for why gender identity and physical anatomy don’t always match. The prenatal hormone theory doesn’t explain everything, but it does suggest that the feeling of “being a man” isn’t purely a social construction. It appears to be shaped, at least in part, by the same hormonal forces that shape the body.
How Cultures Define Manhood
Biology may lay the groundwork, but every culture on earth adds its own definition of what it means to be a man. These definitions often center on rituals, tests, and expectations that boys must meet before they’re recognized as men by their communities.
Among the Bukusu tribe in western Kenya, a 14-year-old boy named Shadrack Nyongesa underwent the traditional circumcision ceremony called sikhebo. The ritual lasted more than a day. He danced through the night wearing cowbells on his wrists while relatives sang songs about courage. His maternal uncle slapped him in the face, told him he looked like a sissy, and warned him not to flinch or cry during the procedure. Elders berated him and pelted him with cow entrails to test his composure. After the ceremony, his life changed immediately: he was given his own hut, freed from domestic chores like fetching water and firewood, and his food preferences were now considered by the women who prepared meals. A follow-up ceremony months later would present him to the community as a full-fledged man.
Other cultures draw different lines. In parts of Arkansas, hunting is so tightly wound into the definition of manhood that not doing it marks a social boundary. In South Africa, boys from the Venda tribe participate in musangwe, a bare-knuckle boxing tradition that begins as young as age nine and serves as both an outlet for aggression and a test of toughness. In Ukraine, children attend summer camps that teach combat tactics, explicitly framed as preparation for boys to fulfill military service. The specifics vary wildly, but the underlying pattern is remarkably consistent: cultures rarely grant manhood automatically. It has to be earned, usually through some demonstration of physical endurance, emotional control, or willingness to protect others.
The Biological Cost of Being Male
One of the less discussed aspects of maleness is that it comes with measurable health vulnerabilities. Male mortality is higher than female mortality in every age group, and no country on earth has a male life expectancy that exceeds the female one. This gap begins at birth. Male infants are more biologically fragile, with higher rates of infectious disease, complications from prematurity, and developmental problems.
Part of the explanation is genetic. Males have only one X chromosome, which means a harmful recessive mutation on that chromosome has no backup copy to compensate. Females, with two X chromosomes, have a built-in redundancy that offers some protection. Beyond genetics, men have higher rates of heart disease and stroke throughout life. The same testosterone that builds muscle and bone also appears to contribute to risk-taking behavior and cardiovascular strain over decades.
Evolution’s Role in Shaping Male Traits
Many of the traits we associate with maleness, from greater upper-body strength to competitive behavior, were shaped by sexual selection over hundreds of thousands of years. Darwin identified two mechanisms: competition between males for access to mates, and female choice of attractive males. Both pressures pushed male bodies and behavior in specific directions. Greater size and strength offered advantages in physical competition. Traits that females found attractive, whether physical or behavioral, were passed on at higher rates.
These evolutionary pressures produced the sexual dimorphism we see today: on average, men are taller, have more muscle mass, denser bones, and broader shoulders than women. But evolution shaped psychology too. The tendencies toward risk-taking, status-seeking, and physical competition that show up across cultures aren’t arbitrary. They’re echoes of selection pressures that favored males who could successfully compete for resources and mates. None of this means men are locked into these patterns. It means the starting conditions have deep roots, and what each individual and each culture does with them varies enormously.