If you’re fortunate enough to find someone you want to settle down with forever, the next question is: How do you achieve happily ever after? According to mathematician Hannah Fry, it may come down to a simple formula.

Fry, who works at the UCL Centre for Advanced Spatial Analysis in London, explains in her 2014 TED Talk and recently released book, “The Mathematics of Love,” that the best predictor of long-lasting relationships is how positive and negative a couple can be to one another.

In her book, she discusses the groundbreaking work of psychologist John Gottman and his team. Over many years they observed hundreds of couples and noted their facial expressions, heart rates, blood pressure, skin conductivity, and the words they used in conversation with their partners.

They discovered low-risk couples have more positive interactions with each other, and high-risk couples tend to spiral into negativity.

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