Mexico didn’t start to phase out leaded gasoline until 1990, and average blood lead levels were at or above 15 μg/dl until then, especially in rural areas. In America, the generation of children born during the era of BLLs that high (1970-1980) was the same generation that later produced the “superpredator” hysteria of the 90s—except that it’s a little unfair to call it hysteria, since many of these kids really were unusually vicious and dangerous.
Mexico, by contrast, had a generation of kids born as late as 2000 with BLLs this high. The fact that violence is endemic 18 years later is no big surprise. In another decade, things should be a lot better.