Why do we keep coming back to the past?

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Let me start with a question. How often has this happened to you? You're arguing with someone you love, whether a partner, a parent, a sibling, and somewhere in the heat of it an old grievance walks back into the room. It isn't from last week. It's from a year and a half ago.

Here is mine. I had promised to be home for a festival - a week before, something urgent came up and I couldn't go. I explained. They understood. There was no fight. As far as I could tell, it was fine.

Eighteen months later, in an argument about something else entirely, there it was: "You didn't come". And I heard myself give the defence I suspect every reader has given at some point. "I came every other time. Every single time. You remember the one time I didn't, and forget everything else."

If you've been on either side of that exchange, and most of us have been on both, you'll know it goes nowhere. One person is fighting about a festival. The other is fighting about arithmetic. Neither is fighting about what's really going on.

If you read me regularly, you know I'm a physicist. For the past few months I've been reading neuroscience, and lately I've been working through Robert Sapolsky's Behave, a doorstop of a book by the Stanford neurobiologist. Its method is simple and humbling. To explain any single human act, Sapolsky asks what happened in the brain one second before it, what hormones were doing hours before, what the person lived through years before, and so on back to evolution. I mailed him and we discussed some points. Then I went looking through the research for an answer to my own small, domestic mystery: why does the one bad thing outweigh all the good ones?

What I found is that three ideas, one from information theory, one from neuroscience, and one from the physics of magnets, explain it better than any of us managed in the middle of a fight.

Good things go quiet

There is a beautiful idea at the heart of information science: the amount of information in an event depends on how surprising it is. If the sun rises tomorrow, you have learned nothing. If it doesn't, you have learned everything.

Now think about my festival record. I came nineteen times out of twenty. By the nineteenth, my arriving was like the sun rising: expected, almost silent. My one absence was the surprise. If you do the calculation properly, that single missed festival carries around fifty times more information than any one of the times I came.

Our brains keep the same accounts. Neuroscientists have found that the brain's reward cells respond most strongly not to good things, but to unexpected good things. Once a kindness becomes reliable, the brain stops registering it. And when something we counted on fails to happen, those same cells fall silent in a way that marks the moment.

Here is the cruel part. The more dependable you are, the more invisible your dependability becomes, and the louder your one failure sounds. She hadn't forgotten all the times I came. They had simply become the background, like the hum of a fridge you only notice when it stops.

Hurt is written in heavier ink

Psychologists have a name for this, the negativity bias, and the evidence for it is overwhelming. A bad event lasts longer in the mind than a good event of the same size. The relationship researcher John Gottman found that couples who stay together tend to have about five good moments for every bad one during conflict. Not one for one. Five to one.

It makes sense if you think like our ancestors. Forgetting where the fruit tree was cost you a meal. Forgetting where the lion was cost you everything. So the brain learned to underline pain. When someone we rely on lets us down, it gets stored with a little red flag attached, whatever we say on the phone that evening.

"It was fine at the time" may be the least reliable sentence in any relationship.

The third answer

In physics there is a phenomenon called hysteresis. Take a piece of iron and hold a magnet near it. The iron becomes magnetic. Now take the magnet away. You'd expect the iron to go back to how it was. It doesn't. It keeps a little of the magnetism, a trace of what happened to it. To understand that piece of iron, it is not enough to know what is acting on it now. You need to know its past.

Love works like that iron. The festival was forgiven, the magnet taken away. But the relationship didn't return to exactly where it had been. It kept a trace.

And neuroscience adds something physics doesn't. Every time we recall a memory, we don't simply replay it. We take it out, and we put it back slightly changed, coloured by whatever we are feeling at that moment. When we're angry, the brain reaches for angry memories. So if that festival had been remembered two or three times in anger since, it had been saved two or three times in anger. The festival that came out that night was heavier than the one that went in. She wasn't exaggerating. The memory itself had changed.

So what was she really saying?

I think it was this: "When it mattered, and something else wanted you, you chose the other thing. Am I really the priority?"

Seen that way, my festival wasn't one day out of twenty. It was the only time the question had been asked under pressure. And counting the other nineteen answered a question she never asked.

I don't believe there is a formula for love. But the science taught me a few things. Stop defending the count; you can win the arithmetic and still lose the person. Don't let your goodness go silent; say the small things out loud, so kindness keeps registering as news. Don't trust "it's fine"; months later, a quiet I've been thinking about the festival I missed can mean more than any defence. And if you're the one reaching for the old wound, ask whether you're angry about that day or about the fear it left behind.

In physics, we say a system with hysteresis has memory. It isn't a flaw. It is exactly what lets a magnet hold on to something after the moment has passed. A love that remembered nothing couldn't hold anything either.

The festival I missed is still there. I've stopped arguing with it. Now I'm just trying to add to the record.

Nishant Sahdev is a physicist at the University of North Carolina at Chapel Hill. He chases ideas that refuse to stay in laboratories, for your favourite Sunday mid-day