Have you ever jokingly told a friend you'll never ditch them because they know too much about you? Well, there could be something in that. New research suggests our inner circles know us so well, they could even be able to predict when we'll die.

A study by the Washington University in St Louis tracked 300 couples, beginning when they were in their 20s up until their deaths.

Right at the start, the couples and their best friends were asked to answer questions about their personalities. The replies were very recently compared to the ages they eventually died. And it turns out their pals made more accurate stabs at when they would pass away than they themselves did.

The study began in the 1930s and quizzed friends that were significant in the young couples' lives, such as their bridesmaids and best men.

Men who were described by their mates as conscientious, reliable and imaginative ended up living longer. However, those seen as less dependable had a shorter lifespan. This could be because they are more likely to take risks.

The women who lived longest tended to be seen as compassionate and emotionally stable by their inner circle. This could be because women with a personality like that are able to fight off depression, anger and anxiety better, all of which could pose physical health risks.

Why are friends so good at sussing you out? Maybe because they can see you objectively and thus pick up on traits you miss.

"Our study shows that people are able to observe and rate a person’s personality accurately enough to predict early mortality decades down the road," researcher Joshua Jackson said.

"It suggests that people are able to see important characteristics related to health, even when their friends were, in the most part, healthy and many years from death."

The study, published in the journal Psychological Science, is one of the longest in psychology.

"It shows how important personality is in influencing significant life outcomes like health," Dr Jackson continued.

"And it demonstrates that information from friends and other observers can play a critical role in understanding a person’s health issues."

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