Many have spent long hours wondering if a marriage is going to last, considering things like love, children, taxes, and the opinion of friends and professionals.

But the best and easiest answer may be this formula:

frequency of lovemaking minus frequency of quarrels

A positive difference predicts marital happiness, a negative one unhappiness.

The formula was derived from a series of studies in the 1970s. One study of married students at University of Missouri-Kansas City found that 28 out of 30 self-described happy couples had sex more than they argued, while all 12 self-described unhappy couples argued more. These results were corroborated by a 1974 study by John Howard and Robyn Dawes, in which all 23 happy couples had a positive score and all 3 unhappy couples had a negative score.

Howard and Dawes describe their findings in "Linear Prediction Of Marital Happiness":

The linear combination discussed above has at least one clear advantage over the commonly used assessment devices: simplicity. Anyone capable of counting and subtracting can use it. Edwards and Edwards (1973, unpublished) consider the difference score predictive; couples with consistently negative scores frequently separate or divorce within relatively short times. If further investigation confirms that the difference score is a reliable predictor, the simplicity of the method might allow couples to use it as-a relatively objective self-monitoring technique. If a previously positive difference shifted to a negative score, the couple would at least be forewarned, and might seek the cause (and cure) of their problems. A limitation of these studies concerns the old saw, "correlation does not imply causation." Not only do we not know direction of causality, we do not know what would happen if the rates of argument or sex were manipulated experimentally or therapeutically. It does not, for example, follow that we can increase marital happiness of people who argue a lot by encouraging them to have intercourse more often--but it would be an interesting technique to try.

We came across this study in psychologist Daniel Kahneman's Thinking, Fast And Slow, in a discussion of the value of simple formulas.

"The important conclusion from this research is that an algorithm that is constructed on the back of an envelope is often good enough to compete with an optimally weighted formula, and certainly good enough to outdo expert judgment," writes Kahneman.

Indeed the best thing about a formula is that it overrides the brain's faulty intuition system. Kahneman also discusses the bad intuition of hiring agents, financial advisors, amateur investors, pundits, and pretty much everyone.

DON'T MISS: 57 Behavioral Biases That Make Us Think Irrationally >