When you've been married a while, you assume you know everything about the other person. But there's a decent chance that's not the case.
According to a new survey, one fifth of married couples have a secret that's big enough it could end their marriage if the other person found out about it.
The most common thing people lie about is cheating, followed by contacting an ex, hiding pornography, hiding debt, using drugs, breaking the law and being gay.
The survey also found the average couple is hiding a total of six secrets between them. And on average, they've been keeping at least one big secret for ten years.
25% admitted they've lied about something important at least once. But 42% of people who are still keeping something a secret are confident they'll never get caught.