Chris Hart: We all love predictions. Tomorrow’s weather forecast, for example. The results of the next election. And how long your relationship will last…

Everyone has an opinion on that one, and so newspapers, magazines and TV endlessly offer suggestions.

But the truth is that even the couple rarely knows. Everyone starts off thinking it will last forever. Then you have your first serious argument. Instantly you’re scared. But afterwards, you reason that arguments make you a more skilled communicator and so you’re closer than before. But that belief’s actually an illusion.

Even trained marriage counsellors struggle to make good predictions, despite carefully basing their judgements on all the facts.

Real experts, in whatever field, know that judgement is unreliable. Instead, they create simple scales to guide their decisions. A famous example’s the “Apgar score” used to decide whether a newborn baby needs help. It’s a simple formula based on five vital signs that can be measured quickly: heart rate, breathing, reflexes, muscle tone, and colour.

The score does far better than a doctor’s judgement in deciding whether to take action, and has saved thousands of babies’ lives.

POSITIVE SCORE

Good marriage counsellors use the same idea, because they know that their impressions of a marriage are no better than the couple’s. So instead of using feelings, or judgement, or intuition, they score behaviour.

One very good approach counts verbal and non-verbal expressions of humour, contempt, anger, and affection. It’s very good at predicting marriage failure or success. But it’s complicated. On this scale, the couples with the best chance of success use humour well, are calm, affectionate, tease lovingly, and are interested in one another, companionable and supportive.

But by far the easiest test is a really simple formula: long term happiness equals how often you make love minus how often you quarrel. You don’t want the result to be a negative number! Because a positive score means happiness, while a negative one predicts misery.

This formula was discovered by psychologists who found that happy couples made love more often than they argued, while unhappy couples argued more than they made love. It really is as simple as that. So simple you can use the formula to monitor your relationship.

Just count your lovemaking and arguments and put them into the formula. Lovemaking means naked between the sheets of course, and an argument means anytime one of you is physically or verbally abusive, has emotional outbursts, refuses to talk, or leaves. Worry if a previously positive score goes negative  and start looking for the reason.

Because couples are OK when they’re loving more than they’re fighting. The other way round and eventually things will go wrong. Because couples with consistently negative scores generally separate or divorce before very long.

Do you argue a lot? I’m afraid the formula doesn’t mean you’ll straightway become happier by getting friskier in bed. You also need to fix the real problems. But it’s definitely worth a try!