These were the most common before no-fault divorce became the common cause so they used to come from court filings. In these days, some of them come from marri

These were the most common before no-fault divorce became the common cause so they used to come from court filings. In these days, some of them come from marriage counseling statistics undertaken during divorce (some states require it before disolving a marriage). There is some specific research from small N groups where N=52 for example (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4012696/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4012696/)