The first 20,000 strange situation procedures: A meta-analytic review.

The Strange Situation Procedure (SSP) was developed five decades ago to assess infant–parent attachment relationships. Although the procedure itself has remained relatively constant in over 285 studies (20,720 dyads) conducted to date, there have been vast sociological changes during this time, and research foci shifts to studying diverse populations. Since its inception, the SSP has also been adopted in over 20 countries. In this meta-analysis, we collate this large body of work, with the objectives of producing reliable estimates of the distribution of the four SSP attachment classifications, assessing temporal trends and geographical differences, and determining if and when distributions are different across various populations. Results revealed that the global distribution of SSP attachment was 51.6% secure, 14.7% avoidant, 10.2% resistant, and 23.5% disorganized. There were no differences in the distribution among mothers and fathers, and no child age or sex differences. We found a temporal trend in which there was less avoidant attachment over time and there were attachment distribution differences between samples from North America versus other regions of the world, particularly Asia, Middle East/Israel and South America. We found higher rates of avoidant and disorganized attachment in populations with sociodemographic risks and higher rates of disorganized attachment in samples where parents had psychopathology and when the child experienced maltreatment or was adopted from foster or institutional care. The implications of these findings for future research and practice are discussed.