Why attachment styles change is not well understood. Waters, Weinfield and Hamilton propose that negative life experiences often cause changes in attachment styles. Their proposal is supported by evidence that people who experience negative life events also tend to experience changes in attachment styles. Davila, Karney and Bradbury have identified four sets of factors that might cause changes in attachment styles: (a) situational events and circumstances, (b) changes in relational schemas, (c) personality variables, and (d) combinations of personality variables and situational events. They conducted a study to see which set of factors best explained changes in attachment styles. The study found that all four sets of factors cause changes in attachment styles. Changes in attachment styles are complex and depend on multiple factors.