Nowadays, everyone knows lead is toxic. Yet for thousands of years, it was treated as a miracle substance. Ancient Romans used it in everything from water pipes to wine sweetener -- an ubiquity that may well have contributed to the fall of the Empire as the civilization's collective IQ decreased due to lead exposure. And in the U.S., lead was also widespread until relatively recently. There's fascinating -- and disturbing -- speculation about how lead exposure in childhood may have created a spike in crime in the 1960s and 1970s. And, as Ben, Matt and Noel discover in tonight's episode, some experts take the theory one step further: Could lead exposure have created a boom in serial killers?