What's the most important milestone in a relationship? Moving in together? Getting married? Having a baby?

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According to a Stanford study, How Couples Meet and Stay Together, the most significant moment in a relationship isn't a special occasion, it doesn't necessarily involve champagne and perhaps doesn't get the recognition it deserves. Rather it is a day like any other that occurs somewhere in the 4th year of a relationship as the likelihood of a couple staying together becomes higher than the likelihood of them breaking up.

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To study couple dissolution rates over time, Stanford implemented follow-up surveys one and two years after the main survey. Using the study's open dataset, we visualized the length of a relationship against the percentage of people reporting to have broken up since the survey was last conducted. The clear indication here is that 5 years is long enough to persuade most of us that our feelings towards our partners is more than infatuation.

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After relationships reach their 5th anniversary, the percentage of surveyed people reporting to stay together becomes higher than those reporting to have broken up.

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To zoom in on this potentially relationship defining trend, we took data for the first 10 years of relationships and plotted the chart again - this time hoping to find more detailed patterns.

The likelihood of a breakup jumps down as the second and again the third years of a relationship pass. But the fourth year of a couple's life is just as likely as the third to end in departure. It's only after a couple reaches the 5th year of their relationship that the likelihood of break up falls sharply.

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The study's data suggests that this passage between years 4 and 5 of a relationship is a significant turning point. And sure enough, just as this time period lowers the probability that a relationship will end, we also see a change in regard to a more familiar milestone - marriage.

Couples are more likely to be married than not after 4 years in their relationship.

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The percentage of surveyed people reporting to be married becomes higher than those

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Taking a closer look at the specific data points recorded for the first 10 years of relationships, it's clear that this passage between years 4 and 5 is significant once more. The likelihood of a couple not being married falls sharply after reaching their 4th anniversary.

The probability of being married increases just as sharply after year 3 of a relationship and somewhere between its 4th and 5th anniversary, the probability of being married actually overtakes the probability of not being married.

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We've also used the data in Stanford's study to investigate the ways in which couples meet and what defines a 'happy' relationship. Check out the investigation here.