A new spanking study (abstract available only) claims that child fussiness at age 1 leads to aggression at age 2 and low IQ at age 3, and all are correlated to spanking. These anti-spanking studies have been criticized by the WSJ and the American College of Pediatricians.
I don't doubt that these correlations exist. If you looked at adults, you would probably find that low IQ and aggressive behavior was correlated with being jailed. But that is not a reason to get rid of jails. It clarifies why we have the jails.