South Africa’s last forensic psychologist has resigned from the police, leaving law enforcement unable to quickly identify serial crime offenders, Sunday Times reported recently. To emphasise the scale of the problem, the article stated: "South Africa ranks among the world’s worst three countries – after the US and Russia – for serial murders and rapes." A similar claim was made in a 2006 article - although the claim only referred to serial murders. Does data back up this startling claim about South Africa? Africa Check delved into it. Lots of unanswered questionsThis fact-check started with a lot of questions. The first: What is meant by "worst"? Did it refer to the number of serial killers and rapists or the number of cases involved? Did the claim consider different population sizes of countries or were absolute numbers being compared? Africa Check contacted one of the journalists who wrote the article, Graeme Hosken, to find out, but he is yet to respond. (Note: We will update this report should he do so.)What is serial murder and rape?The next question was: When are offences counted as serial offences? "There is no one unified definition of serial rape or serial murder," Dr Jackie de Wet, criminology and psychology lecturer at Leeds Beckett University in England, told Africa Check. In the legal system, offenders are charged with multiple counts of murder or rape - not "serial murders" or "serial rapes". "[I]f any one case is prosecuted they will be treated as individual cases of murder and rape," said De Wet. "The concept of serial murder or rape is taken into consideration upon sentencing."FBI definition not universally usedThe absence of an internationally accepted definition of a "serial offence" is why the United States' Federal Bureau of Investigations hosted a serial murder symposium in 2005. More than 130 experts from 10 countries, including South Africa, agreed on this definition for serial murder: "The unlawful killing of two or more victims by the same offender(s), in separate events." Although this definition is widely used, it is not compulsory, said Prof Gérard Labuschagne, who attended the FBI meeting. He used to lead the South African police service’s specialised investigative psychology section. No stats from SA policeThe South African police use the same definition for serial murder as recommended at the FBI meeting. Their specialised investigative psychology section used to keep "a reasonably accurate list" of murder and rape series, said Labuschagne, who left the police service in 2016. We asked the police for the most recent numbers of serial murders and rapes, but they have not yet supplied it. (Note: We will update this report should they respond.) Labuschagne told Africa Check there is no reliable database that compares serial murder and rape on an international scale. "Many of these overseas databases are using media as sources of possible cases," he said. SA: 112 serial killers listed since early 1990sThe Radford/Florida Gulf Coast University serial killer database was set up in 1992 by Dr Mike Aamodt, a psychology professor at Radford University in Virginia. The database contains information on 4 068 serial killers and 11 680 victims since the early 1900s. The information is gathered from websites, books, court documents and government agencies. It was last updated on 23 November 2015.The database currently lists South Africa in third place for the number of serial killers since the early 1900s, with 112 serial killers identified over that time period. The United States (2 743) is first, followed by England (145). Aamodt told Africa Check that his latest data, yet to be published, puts South Africa in fourth place. The database does not provide a yearly breakdown of serial murderers which takes into account different population sizes.Different definitions hamper comparisonDefinitions of crimes vary from country to country. As a result, comparing these statistics has limitations, said Labuschagne.The United Nations Office on Drugs and Crimedefines murder (what they call "intentional homicide") as an "unlawful death purposefully inflicted on a person by another person". However, it goes on to say that country data does not always reflect this definition. They advise that "any cross-national comparison should be conducted with caution" because of different legal definitions, different methods of offence counting and reporting levels.When it comes to serial rape, the research waters are even murkier. There is no database that keeps track of serial rape "because of the lack of legal definitions within all law enforcement framework," De Wet told Africa Check. For example, South Africa’s legal definition of rape is much broader than many other countries’, as it includes any sexual penetration (vaginal, anal or oral) of one person by another regardless of gender. According to De Wet, comparisons of rape statistics are "sensational at best and counterproductive".Conclusion: Experts warn against ranking countries by serial murder and rapeA Sunday newspaper recently reported that South Africa has the third highest serial murders and rapes in the world. Experts warned against these sort of comparisons for a number of reasons. This is because definitions of serial offences differ, as do legal definitions for rape and murder. One serial murder database does place South Africa in third place for the number of identified serial murderers since the early 1990s. However, the ranking does not take into consideration the different population sizes of the countries - which is vital for fair comparisons.This fact-check was researched by Ina Skosana for Africa Check, the continent's leading fact-checking organisation. Read it on their website here.