Consistency is an established indicator of causation in public health research. The consistency of an association between an environmental exposure and a public health outcome is determined by whether the association has been “repeatedly observed by different persons, in different places, circumstances and times” (Hill, (1965).

The association between individual preschool lead exposure and later delinquent and criminal behavior was discovered by Denno (1990) in a study of Philadelphia youths. This association has been confirmed by studies in Pittsburgh (Needleman, 1996; 2002), Cincinnati (Dietrich 2001; Wright, 2008), and Charlotte (Billings & Schnepel, 2018). Stretesky and Lynch (2001) found that USA counties with high air lead levels had murder rates four times higher than counties with low air lead. Nevin (2000) found that gasoline lead emission trends explained 90% of USA violent crime rate variation from 1960-1998, and per capita use of lead in paint and gasoline explained almost half of the variation in USA homicide rates from 1900-1998. The association with violent crime has been confirmed by state and city crime studies (Reyes, 2007; Mielke & Zahran, 2012). Nevin (2007) found that trends in average preschool blood lead explained most of the variation in violent and property crime rates across decades in the USA, Canada, Britain, Australia, New Zealand, Italy, West Germany, France, and Finland. Nevin (2007) also found that differences in gasoline lead exposure associated with city size and differences in lead paint exposure associated with the prevalence of deteriorated and dilapidated housing had an additive impact on USA city murder rates about two decades later. Taylor (2016) found that Australia suburban air lead levels accounted for 30% of local variance in assault rates 21 years later and Australia State data produced comparable results. Boutwell (2017) reported an association between census tract blood lead in Saint Louis and the risk of subsequent census tract violent and nonviolent crime.

This robust research literature shows a clear association between preschool lead exposure and crime at the individual, census tract, city, suburb, county, state, national and international levels. The association occurs with a lag of about two decades, reflecting neurobehavioral damage in the first years of life that especially affects the peak age of offending in the late-teens and early-20s.