This is not specific to serial killers, but violent crime in general.

In the 1990s and 2000s, several researchers found a strong correlation between childhood exposure to lead (Pb) in the environment and violent crime 20+ years later. These include:

• Herbert R. Needleman, University of Pittsburgh Medical Center

• Rick Nevin, in a 1999 study for the US Department of Housing and Urban Development, and a 2006 paper for the National Center for Healthy Housing.

• Jessica Wolpaw Reyes, in a 2007 paper for the National Bureau of Economic Research.

• Howard W. Mielke and Sammy Zahran, Tulane University, in a 2012 paper. (Sorry, no full copy.)

• A layman-friendly MotherJones article is here.

Lead was used as an additive to paint for centuries, and tetra-ethel-lead was used as an anti-knock additive to gasoline for the first 3/4 of the twentieth century. Post WWII, there was both a housing boom and an automobile boom in the US. The population shift of the middle class to the suburbs resulted in the pouring of tons of lead into the atmosphere through burning of gasoline. This urban-flight also had a deleterious effect on the quality of older urban housing, falling into neglect with peeling lead paint. The authors found that 22-23 years after exposure to environmental lead, there was an increase in violent crime, peaking in the 1970s. The dangers of lead in paint were known early. The revelation of the dangers of leaded gasoline were found out much later.

Once lead was removed from paint and gasoline in the 1970s, the US saw a rapid decrease in violent crime, again 22-23 years later, into the 1990s as you observed. While much violent crime tends to be committed by young men in their late teens throughout their 20s, serial killers tend to most prolific in their 30s and 40s. It would be interesting to see if we find an even larger decline in serial killers over the 2010-2030 time frame.

DNA profiling was pioneered in 1986 and with better forensic detection tools available to law enforcement it becomes more likely that a would be serial killer would be caught after their first murder. Especially if they were involved in other crimes. The opportunity to chalk up a large score is diminished.

http://en.wikipedia.org/wiki/DNA_profiling

Here is a lengthy paper (PDF - link saves file to desktop) from Jennifer L. Doleac at the University of Virginia, who is an economist who focuses specifically on crime, exploring the impact of DNA profiling on crime rates from a primarily statistical P.O.V

https://www.aeaweb.org/aea/2013conference/program/retrieve.php?pdfid=129

The DNA databases can also be associated with lower detection rates for first time offenders with law enforcement relying on the databases to produce a match instead of employing old fashion detection methods - however this still correlates with more serial killers being discovered as they are much more likely to have a pattern of offending behaviour.

The people who try to answer questions like this are not usually historians. No significant reclassification has occurred. Donald Lunde's book (Murder and Madness) gives some clues. Better treatment of certain forms of mental illness might be one reason (but again, that's an issue of psychiatric epidemiology, not really a question for historians).

Sociologists, psychologists economists and anthropologists all try to get at both the general trends and the causes for crime. Since there are still quite a few missing people and unsolved murders throughout the U.S., it's possible that serial killing hasn't dropped off as much as we think it has. But if the statistics turn out to be accurate, the causes for the drop have many different hypotheses connected to them and that research is complex and spread throughout many disciplines (including neuroscience and criminology).

One major hypothesis has been the liberalization of abortion laws, since serial killers (according to Lunde, a psychiatrist who specialized in the study of serial killer) typically have both biological and early childhood traits that set them on the path to their crimes. One can set up dozens of hypotheses about what has changed, but certainly the Donohue-Levitt hypothesis has some evidence to back it. Donohue and Levitt are both economists, and specialize in the ways that crime correlates to economic changes. They undertook a study of how legalized abortion and a reduction of unwanted children led to a drop in crime 18 years later and persisting into the present time. Donohue has also spent time teaching in law schools, so we can add legal expertise to the list of disciplines that study this question.

Statistically speaking, is there really a difference in the number of serial killers between these two periods or as the OP put it, does it just seem that there are less?

Is there some sort of stat that says there was a drop off in the 90s? I know pop culture has given more publicity to some of the older serial killer cases but that does not necessarily mean there was a drop off. John Douglas is the former head of the FBI's serial crime investigation unit and author of a book about serial killers called Mind Hunter. Douglas conservatively estimated that there are 35-50 serial killers active in the US at any given time. This makes it seem like there wasn't a drop off in serial killers so much as a drop off in publicity about them.

Yeah the way this question is phrased leads me to believe that this really should be asked in /r/SocialScience or /r/Psychology. If you wanted to know the history of classifying serial killers, notorious serial killers throughout history, or the history of catching serial killers then this would be the subreddit for you. As it stands, most of the answers to the question are either unsubstantiated or only tangentially related to your question.

A lot of the difference between early serial killer cases and modern cases is the classification. A lot of people now classified as serial killers would not have been identified as such in the 70's and 80's, and didn't receive the designation until long after their they were imprisoned. However, as more people's past crimes come to light through modern detective techniques, more of these people's past murders have come to light. The point is, maybe modern investigators haven't found out about all a serial killers murders just yet to classify them. It takes many years to be able to conclusively call someone a serial killer.

This thread is a disaster area of speculation and highly contested hypotheses and I do not believe that this sub is the best place to discuss this, especially as this alleged drop off really only manifests itself within our no-go era of the past twenty years. This is really more suitable for /r/AskSocialScience and I am pulling it.

Freakenomics suggested that part of the reason that crime decreased was because abortions became easily available after Roe V Wade, and because their we less unprepared parents, and everything that can go a long with that (poverty, less education, less resources ) their were less criminals produced. They used statistical information form the US and Romania to show the link between the availability of abortions and crime rates over the next 30 years.

It looks like there has been a rash of serial killers all over this thread. Wielding Occam's Razor, they have slashed everything else away and decided that the answer is "lead" or "abortions".

Actually I think that even those answers assume too much. There's got to be a simpler answer.

While the lead theory may seem plausible, correlation does not equal causation. To build off of athosghost's thought process, I argue that modern forensics along with media coverage are the two major factors in a perceived or real decrease in serial killings or killers. In previous decades police departments did not have all the wonderful forensics that we have now (I know shocker) and murder investigations would often run cold after sparse witness accounts were exhausted. The media was also often used to flush out leads and additional witnesses. This is not a tactic used as often today to dissuade copycats and unnecessary public panic. In a time before computers, security cameras, credit card records, etc. police departments had a lot less to work from AND when you can only see part of a picture everything starts to look the same. "Oh hey look, another mid 20s female stabbed in her home, must be a serial killer!" OR females in their mid 20s have a higher rate of domestic violence and chance of being killed by a husband or boyfriend. Said boyfriend/husband says he was at the bar that night and several regulars say they drank with him…bing-bang-boom, a serial killer on the loose (when really it was just the boyfriend/husband). Modern forensic techniques can now analyze DNA, computer forensics and countless digital footprints we leave without knowing to identify the variance from one murder to another. So, big summary; a decrease in serial killers is likely perceived rather than actual due to our advancements in science and forensics. I don't think a reclassification occurred, but the criteria may have become clearer.

DNA testing in 1982?

There is much more information at www.ricknevin.com about the link between childhood lead exposure trends and murder rates in particular, and other categories of violent and property crime in the USA and in many other nations. Recent news stories and independent analysis includes: http://jjie.org/analysis-is-lead-exposure-the-secret-to-the-rapid-rise-and-fantastic-fall-of-the-juvenile-crime-rate/106841/ http://cen.acs.org/articles/92/i5/Crimes-Lead.html http://www.bbc.com/news/magazine-27067615

All of the data about lead paint seems to totally discount advancements in psycopharmacology and a more widespread acceptance of mental illness. I would credit the decline in serial killing sprees more to a general cultural shift, than a change in the literal atmosphere.