The Winner Effect: How Winning Rewires Brains and Hormones

Winning a fight changes something inside the winner, making the next fight easier to win. This self-reinforcing cycle, known as the winner effect, has been documented across a remarkably wide range of species, from fish and insects to mice and humans. The basic phenomenon sounds simple, but the biology behind it involves testosterone surges, rewired brain circuits, stress hormones, and reward pathways that together create a kind of neurochemical momentum. And that momentum has a dark twin: a loser effect that can be just as powerful in the opposite direction.

How Winning Rewires Hormones

The hormonal backbone of the winner effect is testosterone. After a victory in a competitive encounter, testosterone levels spike, and that spike does more than just reflect what happened. It actively pushes the animal toward bolder, more aggressive behavior in its next confrontation. Research in California mice has shown that this process is cumulative but also requires the right combination of ingredients. Males that won three separate territorial disputes and experienced a testosterone surge after each one developed a robust winner effect. Males that had the wins without the hormone surge, or the surge without the actual wins, showed only a partial effect. Both pieces matter, and they appear to operate through separate channels in the brain.

In humans, the picture is similar in outline, if messier in detail. A study of men and women competing in laboratory tasks found that testosterone reactivity to competition mediated the effect of winning on subsequent aggressive behavior in men, though not in women.¹ That sex difference is important and comes up again later. What matters here is that testosterone is not merely a marker of dominance; it is a causal ingredient. When the hormone surge is paired with the social experience of winning, the two reinforce each other and create an effect that exceeds what either contributes alone.²

What Changes in the Brain

Testosterone does not just float around in the bloodstream making an animal feel tough. It reshapes the brain’s sensitivity to itself. In California mice that won territorial fights, researchers found that the expression of androgen receptors increased in a brain region called the bed nucleus of the stria terminalis, which helps control social aggression. That increase happened regardless of whether the mouse fought on its home turf or someone else’s. But in the reward-related parts of the brain, specifically the nucleus accumbens and the ventral tegmental area, androgen receptor expression went up only when the mouse won at home.³ This distinction between aggression circuits and reward circuits is one reason context matters so much for the winner effect, a point explored further below.

More recent work has drilled deeper into the neural circuitry. A 2024 study published in Cell used optogenetics to artificially activate the connection between the posterior amygdala and a part of the hypothalamus involved in aggression. Stimulating that pathway replicated the cascade of changes that normally occurs during repeated winning, essentially manufacturing a winner effect in the lab without an actual fight.⁴ This kind of causal evidence is particularly convincing because it shows the winner effect is not just a statistical tendency or a hormonal correlate. It is a specific pattern of neural plasticity that can be triggered by the right signals in the right circuits.

The Loser Effect and the Stress Hormone Trap

If testosterone powers the winner’s upward spiral, the stress hormone corticosterone (cortisol in humans) anchors the loser’s downward one. After losing a fight, animals experience rising glucocorticoid levels, and those hormones do not simply fade. In rats, researchers found that injecting corticosterone directly into the brain of the emerging subordinate after a social defeat was enough to lock in that animal’s low rank for the long term.⁵ The stress response did not just reflect the loss; it cemented it.

From an evolutionary standpoint, both effects make sense. An animal that just won a fight has received fresh evidence that it is strong relative to its opponents, and escalating future encounters is a reasonable bet. An animal that just lost has received the opposite evidence, and retreating from aggressive opponents is the safer play. Modeling work suggests that adaptive behavior generates clear winner and loser effects because individuals benefit from updating their self-assessment after each encounter, escalating when they have reason to expect victory and backing down when they do not.⁶ The problem is that these updates can overshoot, pushing winners to overconfidence and losers to unnecessary submission.

Why Home Turf and Audiences Change Everything

The winner effect does not emerge in a vacuum. Where a fight happens and who is watching both shape its strength. In California mice, winning at home and having prior winning experience each boosted future fighting ability independently. But combining the two produced a result greater than the sum of its parts, suggesting a synergistic interaction. Animals that won on their home territory developed the full, robust winner effect, whereas those that won only on unfamiliar ground developed a weaker version.⁷ The neural data mentioned earlier fits neatly here: the reward-pathway changes triggered by winning appeared only in home-territory fights, not away fights. So the brain literally processes a home win differently from an away win.

Audiences add another layer. In Japanese quail, the presence of a familiar mixed-sex audience during a fight altered both the testosterone response and the animal’s long-term competitive trajectory. Losers that fought in front of an audience failed to show the usual post-fight testosterone increase, while losers that fought in private still got a hormone bump. The implication is that losing publicly is hormonally worse than losing privately, as if the social stakes of the defeat suppress the very hormonal recovery that might help the loser bounce back.⁸ Anyone who has felt the sting of a public humiliation lasting longer than a private failure can probably relate.

From Individual Fights to Group Hierarchies

Zoom out from a single pair of opponents, and the winner and loser effects start to look like an engine that builds social hierarchies from the ground up. Simulation models have explored how simple rules about winning and losing can produce the kind of stable pecking orders seen in real animal groups. One such model showed that when aggression intensity was set high and the winner-loser effect was active, the simulated group produced hierarchies with the high linearity typical of hen flocks, meaning a clean, top-to-bottom ranking rather than a tangled web of dominance relationships. When either the winner-loser effect or the spatial structure of the group was removed, the resemblance to real hierarchies dropped sharply.⁹

A separate analytic model, approaching the question from an evolutionary angle, showed that winner and loser effects can evolve through repeated interactions among small groups of animals drawn randomly from a population. The model also predicted that the stronger the combined winner-loser effect, the more likely it is that a stable hierarchy will emerge. But it came with a caveat: if an individual’s inflated self-assessment outstrips its actual fighting ability, the cost of fighting goes up with the overestimate.¹⁰ A winner effect that runs unchecked can lead to costly overconfidence, a thread that becomes significant when we get to human behavior.

The Winner Effect in Human Competition

The leap from mice and crickets to human psychology is always tricky, but there is direct evidence that something resembling the winner effect operates in people. In a controlled study where participants competed on two consecutive days, testosterone reactivity on the first day, specifically the net testosterone increase in those randomly assigned to win, predicted their actual task performance the next day.¹¹ This is not just about motivation or mood. The hormonal response to a prior win changed how well people performed in a subsequent competition, which is the core definition of the winner effect translated into a human context.

A biopsychosocial model proposed by organizational psychologists links these testosterone fluctuations to outcomes relevant in workplaces and competitive institutions. The model predicts that winning and losing status in intrasexual competitions should affect not only aggression but also empathic accuracy, the ability to read other people’s emotions, and personal distress in response to others’ suffering.¹² In other words, winning may make people not only more competitive but also less attuned to the emotional states of those around them. For leaders riding a streak of victories, that combination could be potent and not entirely positive.

Sex Differences in the Hormonal Response

The evidence on sex differences is still evolving, but one consistent finding is that testosterone’s mediating role in the winner effect looks different in men and women. In the study described earlier, the link between winning-related testosterone changes and subsequent aggressive behavior held for men but not women.¹³ This does not mean women are immune to winning streaks or losing streaks. It means the hormonal pathway through testosterone may not be the main mechanism for women, and other systems, perhaps cortisol-related or entirely different, may play a larger role. Researchers have been slower to map those alternative pathways, which is a gap worth noting. Much of what we know about the winner effect’s biology comes from male subjects, both animal and human, and the field is still catching up.

When Winners Take Too Much Risk

The winner effect evolved in contexts where fights are relatively brief and the stakes are physical. But humans have transplanted competitive dynamics into domains like financial markets, where the consequences of overconfidence can ripple far beyond the individual. An experiment that administered testosterone gel to young men before they played an asset-trading game found that testosterone shifted their investment toward riskier assets and made them more optimistic about future price changes.¹⁴ Cortisol, separately administered to a different group, also increased risk-taking, though it appeared to affect risk preferences more directly rather than through optimism.

This is where the evolutionary model’s warning about overestimation becomes relevant. In a territorial fight, an animal that overestimates its strength and picks a fight it can’t win pays with injuries. In a financial market, a trader on a hot streak may experience testosterone surges that push them toward bigger, riskier bets. If those bets pay off, the cycle intensifies. If enough traders experience this simultaneously, the aggregate effect could amplify market bubbles. The winner effect, in short, may be one of the biological mechanisms underlying the well-documented tendency for human risk-taking to increase during booms and only correct itself through painful losses.

Can the Loser Effect Be Reversed?

If the loser effect is partly hormonal and partly neural, it stands to reason that it might be reversible. Research in insects has identified specific neurochemical levers. In fruit flies, the loser effect produced by social defeat made the animals avoid fights altogether. But boosting serotonin levels in a specific set of neurons was enough to restore their willingness to fight. The effect was specific: serotonin itself, not other neurotransmitters released by the same neurons, was responsible for reversing the loser state.¹⁵

In crickets, the story involves dopamine instead. After social defeat, crickets become submissive and refuse to engage. Researchers found that aggression could be restored prematurely by administering a dopamine receptor agonist, a compound that mimics dopamine’s effects. This worked even in crickets whose neurotransmitter stores had been artificially depleted. The aggression-promoting effect was blocked by a dopamine receptor antagonist but not by an octopamine receptor antagonist, pinpointing the relevant pathway with unusual precision.¹⁶

Whether serotonin or dopamine manipulation could meaningfully address the psychological consequences of repeated defeat in humans is a question that remains wide open. Humans already use drugs that target both systems for depression and related conditions, many of which involve social withdrawal and learned helplessness that at least rhyme with the loser effect. But the distance between boosting serotonin in a fly’s neurons and prescribing an SSRI to a person who has internalized a pattern of failure is enormous. The insect work is valuable because it identifies discrete neurochemical switches that can flip the loser state off, which tells us the state is not permanent and not purely psychological. What it cannot tell us is how those switches map onto the far more complex human brain.

Why the Winner Effect Is Not the Same as Confidence

A common misconception is that the winner effect is just a fancy name for confidence. It is not, or at least not only. Confidence is a psychological state that can come from many sources: encouragement, preparation, self-talk, or simply being in a good mood. The winner effect is a specific biological cascade triggered by the experience of winning a competitive encounter. It involves measurable changes in hormone levels, receptor expression in targeted brain regions, and plasticity in neural circuits connecting the amygdala to the hypothalamus. You can be confident without having recently won anything, but you cannot have a winner effect without a prior victory.

This distinction matters because it means the winner effect has a shelf life and a context dependency that ordinary confidence does not. The hormonal and neural changes appear to be strongest shortly after a win and in similar competitive contexts. A mouse that wins a territorial fight does not become universally bold; it becomes specifically more likely to win the next territorial fight, especially on its own turf. Similarly, the human evidence suggests that performance boosts carry across consecutive days of the same type of competition, not necessarily across unrelated life domains. The winner effect is more like a tuned instrument than a personality trait: it plays louder in the key it was tuned in.

The species diversity of the phenomenon is also worth pausing on. The winner effect has been documented in fish, crustaceans, insects, birds, rodents, and primates, which suggests it is deeply conserved in evolution and probably quite old.¹⁷ That breadth makes it hard to dismiss as a quirk of any one type of nervous system. Whatever the winner effect is doing, it has been useful enough for natural selection to maintain it across hundreds of millions of years of evolutionary divergence. The specific neurochemical systems recruited differ across species, serotonin in flies, dopamine in crickets, testosterone in mice and men, but the functional logic is consistent: win now, fight harder later.