Sociologists and criminologists have not reached consensus on the explanation for the dramatic decrease in the city's crime rate. Some attribute the phenomenon to new tactics used by the NYPD, including its use of CompStat and the broken windows theory. Others cite the end of the crack epidemic and demographic changes, including from immigration. Another theory is that widespread exposure to lead pollution from automobile exhaust, which can lower intelligence and increase aggression levels, incited the initial crime wave in the mid-20th century, most acutely affecting heavily trafficked cities like New York. A strong correlation was found demonstrating that violent crime rates in New York and other big cities began to fall after lead was removed from American gasoline in the 1970s. Another theory cited to explain New York City's falling homicide rate is the inverse correlation between the number of murders and the increasingly wetter climate in the city.