Didn't forget. This was pretty much settled science well before the internet became a big deal, thus...
When you try to search for this on the net, you get tons of shit on investigating details of this, such as comparing illegitimacy/crime increases during different decades, with the link between illegitimacy and crime treated as a given, not something to be supported.
Study: Crime rates linked to out-of-wedlock births
"ROLE OF
SOCIAL STIGMA
While Kendall and Tamura found that more out-of-wedlock births are associated with higher subsequent crime rates over the last 45 years, they found just the opposite for years prior to 1965. In the 1940s and 1950s, small increases out-of-wedlock births actually correlated to lower subsequent murder rates.
Why did the association change? Kendall and Tamura argue that changing social attitudes toward unwed motherhood are the primary reason. In the 1940s and 1950s, social stigma against unwed mothers was intense, and as a result most pregnant couples ended up getting married—even couples who were very poorly matched. Some of those marriages would have been so bad that the children involved might have been better off—and less inclined to commit crimes as adults—if their parents had never married. So a moderate increase in out-of-wedlock births during this period meant that fewer poorly matched couples were forced to marry, thus exposing fewer children to damaging marriages.
In the 1960s and 1970s, however, social attitudes toward unwed motherhood changed rapidly and rates of out-of-wedlock births skyrocketed. Many of the marriages foregone during this period would have been relatively well-matched couples, Kendall and Tamura argue. “The children of these higher-quality matches are worse off because of their parents’ failure to marry, and have higher risks of adult criminality,” they write."
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Study: Crime rates linked to out-of-wedlock births