releases the neurochemical oxytocin
when you trust someone, and it feels good. Oxytocin wires your brain to seek opportunities to trust to get more of that safe feeling.
But when someone betrays your trust, your brain releases cortisol, which feels bad. Cortisol signals pain, and wires you to avoid whatever caused the pain. Everything associated with a betrayal of trust tends to trigger your cortisol alarm in the future.
When you trust, you risk ending up with that unpleasant cortisol feeling. But when you withhold trust, you miss out on that pleasant oxytocin feeling. So in every moment you must choose whether you are better off trusting or withholding trust. It's a daunting task, but your brain evolved to do it.
Your three brains work together when you decide who to trust - your reptile brain, your mammal brain, and your cortex.
Your reptile brain evolved to avoid pain. It is always scanning for cues associated with past pain.
Lizards make social decisions with a few simple rules. If it's bigger than you, run. If it's smaller than you, try to eat it. If it's about the same size, try to mate it.
A lizard starts running the moment it cracks out of its shell, and if it doesn't run fast enough, its parent may eat it (wisely recycling weaklings into new siblings instead of leaving them for predators). Reptiles do not trust other reptiles. They do not make oxytocin, and they do not seek survival by bonding with other reptiles.
Your mammalian brain evolved to seek the safety of the herd. Mammals release large quantities of oxytocin at birth. That bonds an infant to its mother, which protects it from wandering off and getting eaten. Gradually, the mammal brain wires itself to feel attachment to a herd or pack or troop rather than just its mother. A herd animal releases cortisol when it can't see at least one other herd-mate.
But some herd-mates are more trustworthy than others. Stronger gazelles push their way to the center of the herd where it's safer from predators, leaving the weak ones exposed to predators at the fringes. But herd animals stick with their herd regardless, because they get eaten by a predator if they leave. Herd animals don't rehash this decision all the time. They just respond to the neurochemistry that makes familiars feel good and isolation feels bad.
The bigger a mammal's brain, the more social information it processes. Primates use their big cortex to constantly re-negotiate their social alliances. The cortex works by finding patterns in the chaos of sensory inputs. It uses patterns experienced in the past to make sense of the present and project into the future. But the past is not always a good predictor of the future, so you can never fully protect yourself from the pain of betrayed trust.
Baboons choose their friends carefully. To a baboon, friendship means coming to each other's defense when attacked. If a baboon sends out an alarm call and their trusted ally does not come, the alliance is over. The baboon will seek new allies rather than defend a comrade who does not reciprocate. Baboons cement alliances by grooming each other's fur. The sensation of touch triggers oxytocin, which wires them to have that good feeling in the presence of the other.
Chimpanzees also build trust with grooming. Lying
still with another chimp's hands on you is risky in the chimp world, because chimps sometimes lose their temper and bite. Many chimps have lost a finger or toe at the hands of a troop-mate. They reconcile with more grooming. Imagine all the trust it takes to let another get that close! Lots of oxytocin is triggered by touch, and by neural circuits built up from past bonding experience.
Your ability to analyze the trustworthiness of others evolved for a reason. When the lion laid down with the lamb, usually the lamb got eaten. So the world was left to lambs that made better decisions. Animals that formed sturdy alliances had more surviving offspring, and the capacity to trust wisely was naturally selected for.
Your ancestors had less opportunity to decide for themselves who to trust. They learned simple rules at a young age: trust this, don't trust that. Of course the rules were wrong sometimes, but people weren't always free to analyze the patterns for themselves. They just lived with that painful cortisol feeling. Today, you have the freedom to decide for yourself when your trust is merited, and when it is likely to be betrayed. That freedom brings a lot of uncertainty. We can celebrate our freedom to call the shots ourselves, and live with the consequences.
Read more about oxytocin and the other "happy chemicals" in Chapter 2 of my book, I, Mammal: Wy Your Brain Links Status and Happiness.