In the U.S., the number of serial killings reached 692 in the 1980s, compared with 534 in the previous decade, the 1970s. (See the first reference link). According to a study by professors, Ronald Holmes and Stephen Holmes, this increase came as a result of a rise in the incidence of violent crime, more generally. It was also caused by the increased reporting of such cases which created a "pandemic fear" that everyone in society was at risk of being murdered by a serial killer. (See the second reference link).
In the 1990s, however, there was a slight decline in this figure (from 692 to 614 people) and one contributing factor is the increased prison population in the U.S. This kept more violent predators behind bars and protected people on the streets. The development of DNA profiling and forensic databases also contributed to this decline because it stopped many potential serial killers from amassing more than one victim. (See the third reference link).