In their study Skodol et al. identify borderline personality disorder as a chief cause of persistent depression. Over 15% of adults with depression in a national survey had persistent illness over the next 3 years. Borderline personality disorder was estimated to account for over half of these cases of persistent depression. Other comorbid disorders associated with persistent depression were schizoid and schizotypal personality disorders, any anxiety disorder (the strongest axis I disorder predictor), and dysthymic disorder. Substance abuse was not a significant predictor. A complementary 2010 study by Morey et al. found that borderline personality disorder diagnosed when patients are depressed persists whether or not the depression remits, which indicates that the personality disorder diagnosis is an enduring trait and not simply another manifestation of depression.