On a rainy afternoon in Manhattan, a young woman on the subway found herself tightly clutching her bag, heart pounding. She had just read a news alert about a nearby incident and instinctively shifted closer to fellow commuters who looked like they worked nearby—suits, badges, tote bags from familiar stores. When a man entered speaking another language and glancing around nervously, she stiffened, unsure why. It wasn’t overt fear, but something subtle—a reflex. That moment, trivial as it seemed, captured something deeply human: under stress, we seek safety in familiarity and grow wary of difference.
Stress does not just affect our heart rate or sleep. It reshapes the very way we connect—or disconnect—from other people. Decades of research have tried to pin down whether stress makes us more aggressive or more cooperative, but the answer is more nuanced than we once believed. The truth, as new research now shows, is that stress does both. It brings us closer to our own kind while turning us against those we perceive as outsiders.
This dual effect stems from how our bodies process acute pressure. The stress response isn't simply about panicking or escaping danger. Biologically, it’s driven by two main neurochemicals: cortisol and noradrenaline. These substances don’t just get our hearts racing—they quietly shape how generous or combative we become in social settings. And their influence is strikingly specific.
Cortisol, often vilified as the hormone that wreaks havoc on our immune system and causes weight gain, actually has a surprising social side. When released during stress, it prompts us to seek connection and support from those around us—our inner circle. Think of a firefighter unit after a long, dangerous shift. Exhausted and wired, they aren’t just decompressing over coffee. Their bonds, strengthened under pressure, become a kind of armor. They laugh harder, trust deeper, and even grieve together more openly. That’s cortisol at work, nudging people to unite and protect their tribe.
But noradrenaline, which rushes through the brain like a siren during moments of perceived threat, doesn’t share that inclusive instinct. It prepares us for conflict, heightens vigilance, and primes us for defense. If cortisol whispers “lean in,” noradrenaline shouts “watch out.” In practical terms, it means when we're under pressure, we may instinctively treat outsiders with suspicion or even hostility—even if there's no rational reason. It’s a neurological tug-of-war, where unity and bias are born from the same emotional fire.
One of the clearest examples of this emerged from a controlled experiment in which participants were given either cortisol or noradrenaline and then placed into group-based economic games. The game allowed them to allocate money among members of their group or punish rivals—even at personal expense. The findings were telling. Those under the influence of cortisol acted generously, but only toward members of their own team. Meanwhile, those fueled by noradrenaline became more cutthroat, actively trying to damage rival groups’ outcomes.
This isn’t just lab science—it plays out daily in real life. During the early days of the pandemic, many communities showed incredible solidarity. Neighbors delivered groceries to the elderly, parents formed learning pods, and local businesses supported frontline workers. Stress brought people closer, made them more generous—but mostly within familiar circles. At the same time, tensions flared toward those perceived as “other.” From hate crimes to neighborhood tensions, the division between “us” and “them” became painfully visible.
What’s most sobering about this dynamic is how deeply it’s rooted in our biology. Parochial altruism—the term researchers use to describe people helping their ingroup while being hostile to outsiders—isn’t just a cultural artifact. It’s baked into our stress response. This behavior may have once served an evolutionary purpose. In early human tribes, bonding tightly with your group while guarding against others could mean survival. But in our interconnected modern world, the same instincts can deepen social divisions and even fuel discrimination.
Understanding this helps explain everyday behavior too. Imagine a mother navigating a crowded airport with her young child after a missed connection. She might smile at a fellow parent juggling a stroller, but scowl when someone cuts the security line. Or consider how fans at a high-stakes sports game behave. In the heightened emotional stress of competition, cheering becomes tribal, and jeers fly not because people are cruel, but because stress is priming their sense of group identity.
There’s also an important takeaway here for our mental and physical health. High stress doesn’t just make us more reactive; it subtly shapes our social filters. People under chronic stress may gravitate more tightly toward their own communities, while their openness to diversity decreases. This isn't necessarily about prejudice—it's a kind of emotional economy, where trust and goodwill are reserved for the familiar. Left unchecked, this pattern can wear down broader social cohesion.
Yet, knowing that this dynamic exists also offers hope. Stress responses aren’t destiny—they're tendencies. Being aware of how our neurobiology works can make us more mindful in high-pressure situations. Health professionals, for instance, have long observed how support groups help patients not just cope with illness but build resilience. These groups provide the closeness our cortisol systems crave. But they also offer a safe space to challenge defensive reactions, to open up to people from different backgrounds who share the same struggle.
In business leadership, understanding these dynamics is crucial too. When companies go through mergers, layoffs, or intense competition, stress levels soar. Leaders may see team cohesion increase but also notice rising tensions between departments or branches. Creating deliberate opportunities for intergroup collaboration and empathy—such as rotating team responsibilities or shared training programs—can counterbalance the natural “us vs. them” reflex.
Education and parenting benefit from this knowledge as well. Children facing academic or social stress often show more loyalty to close friends and less openness to outsiders. Encouraging inclusive play, mixed-group projects, or collaborative challenges helps foster broader social empathy. Rather than punishing exclusion, we can guide young people through it by recognizing the protective instinct beneath the behavior.
Even in policy design, stress-based group behavior has implications. Natural disasters, public health crises, or even political upheaval place entire populations under pressure. The instinct to rally around familiar flags is powerful, but so is the potential for division. Public health campaigns, community outreach, and even city planning should consider how people’s sense of safety and identity can be strengthened without reinforcing exclusivity.
At its core, this isn't about choosing cooperation or competition—it’s about acknowledging both. Our stress response contains multitudes. It pushes us to connect and to protect, to care and to defend, often at the same time. There’s beauty and risk in that. As individuals and communities, learning to channel these instincts wisely may be one of the most profound forms of emotional intelligence we can cultivate.