A few years ago, I was co-presenting on evidence based policing at the CAPE (Canadian Association of Police Educators) conference, when we got to the part I dread: the Q&A session. Anyone who knows anything about policing knows that police officers make undergraduates look thirsty for attention when it comes to jacking one’s hand up to ask questions that might potentially expose the fact they weren’t listening. In other words, I was prepared for dead silence. So, imagine my surprise when a nice young man stood up and asked me if we had ever learned anything that had contradicted what people generally thought to be true. I thought for a second. Then I remembered a paper that had come out a few years previously: “Tracking the Evidence for a ‘Mythical Number’: Do UK Domestic Abuse Victims Suffer an Average of 35 Assaults Before Someone Calls the Police?” As I explained, this ‘stat’ was based on a highly flawed study, as the authors detailed:
It turns out that a few people in the room had heard this number and thought it was based on some known reality rather than on one deeply flawed study.
Flash forward a few years and I’m sitting in an office at London HQ with my colleagues, Maria and Dave*, discussing missing persons data and tossing ideas around. We’re talking about how the overwhelming majority of missing persons calls are ROO (‘returned on own’) or otherwise ‘located’ fairly quickly, when Dave jokingly mentions the old ‘don’t report someone missing for 24 hour rule’ frequently cited on television. In jest, I reply we can’t make people wait 24 hours to report because the ‘48 hour rule’ says if police don’t begin investigating, investigators will lose leads and reduce case solvability. And that’s when it hits me: where the hell did that number come from? And why 48? Why not 24? Why not 72? Why not even 96?
I went looking in the research literature and here’s what I found:
1. No actual tests of 48 as the number for maximal solvability. One pair of authors makes a similar point: “Although the first 48 hrs have not been a specific focus of empirical research …” (Reale and Beauregard 2019).
2. Despite #1 above, several authors directly cite the 48 hour rule as an industry standard: “It is generally recognized that the critical time interval for identifying suspects, witnesses and evidence is the first 48 hours after a homicide is reported” (Carter and Carter 2015). “The first 48–72 hours of an investigation are crucial to its success as the loss of vital information and evidence accelerates after this time frame” (Dean and others 2008). Sewell (1994) says, “Law enforcement training has always suggested that the first 48 hr are the most critical to the successful conclusion of a homicide investigation.” And, “because the first 48 hours of a homicide investigation are critical, the project identified critical tasks in the first 48 hours of the investigation that increase the probability of a clearance” (Carter 2013).
3. Other authors unquestionably accept the 48 hour rule as a standard and talk about how agencies develop best practices around this standard: “[an agency studied recommends police] brainstorm about future directions for the investigation. This process brings out different perspectives and, if repeated several times during the first 48 hours of the investigation, can keep investigators “fresh” to generate further ideas” (Richardson and Kosa 2001). Or, about this rule informs police practice: “It is commonly noted that the initial phase of a follow-up investigation occupies a special significance to homicide detectives … Dubbed “the first 48 hours,” investigators race to amass as many leads as possible in hopes of quickly identifying a suspect before the trail goes cold.” (Reed and others 2019).
4. In support of the contention there is a special ’48 hour rule’ to solving cases, Alexander and Wellford (2017) cite previous research by one of the authors (Wellford and Cronin 1999) as having identified this period as "critical to closure".
The Wellford and Cronin study examined various characteristics of 798 homicide cases that had been conducted in four large U.S. cities during the period of 1994 to 1995. The authors clustered case closings into one of the following categories:
1 day
2 days to 1 week
1 week to 1 month
1 month to 6 months
6 months to 1 year
Missing
As can be seen there is no specific cut-off for 48 hours; rather the 48 hours window is lumped into the 168 hour window (1 week).
There’s also a small problem with the numbers. The totals for all cities combined are:
1 day:169 cases
2 days to 1 week: 125 cases
1 week to 1 month: 99 cases
1 month to 6 months: 126 cases
6 months to 1 year: 28 cases
Missing: 19 cases
In other words, even if we accepted there were 294 cases at the 48 hour mark (169+125), the cases from the 168 hour mark and over total 272 cases – a difference of 22 cases. This is hardly the stuff of "proof".
Lastly, Wellford and Cronin also blow apart the 'first 48 rule'.. They themselves note that "City C solved the fewest cases in one day. The other three cities solved 32% to 40% of their homicide cases within 24 hours. City C only solved 8.1% of its cases in the first 24 hours after the homicide. However, City C solved more of its homicide cases between 1 and 6 months than the other three cities."
Aside from the fact that it's generally fun for academic geeky types to trace these types of myths, there are reasons why it's important more generally to debunk these things. Paramount among these is the sheer prevalence of belief in the magical 48 hours, thanks to several so-called fictional and 'reality'-based TV shows. Try Googling "48 hours and police" and you'll see what I mean. One of the first hits to come back ties this rule to the Sherman homicide case in Toronto, using it in a critical way to cast doubt on the initial police investigation. Similar other stories are easily found. That's why it's good to some times sit back and think, 'where the hell did that actually come from?'
*Thanks to Maria and Dave for yet another interesting and fun discussion.
** Thanks also to Stacey Pearson, who asked around and was told the number comes from Vernon Gerbeth's Practical Homicide Investigation. I checked two different versions of this text and was unable to find it referenced by Gerbeth. I searched both versions by looking for both 48 and 48 hours.