The chicken-egg problem also applies to studies on happiness and social relationships. Many cross-sectional studies show strong correlations between happiness and successful friendships, marriages, and family relationships. Longitudinal studies, however, demonstrate that the tendency toward happiness precedes strong relationships. In other words, happy people are more likely to get married, have successful marriages, and even have more friends. For example, in an Australian study carried out over fifteen years by Gary Marks and Nicole Fleming, people who scored high on happiness scales were more likely to be married in the following years than those who did not. Similar results have been found with German and American samples. Moreover, a 1989 study published by Bruce Headey and Ruut Veenhoven showed that happiness levels predicted quality of marriage. Over a six-year period, people with higher levels of happiness earlier in the study were more likely to have a happy marriage later in the study.