Adventures on the Back of the Envelope, Part II: Warring Definitions of “Probable Cause”

While watching “Win Win” this evening (good movie, BTW), I decided to run another Criminal Procedure back-of-the-envelope drill. This time, I wanted to see if I could track the utilization of the various definitions of “probable cause.”

Among these definitions, courts have stated that probable cause exists “when the facts known to the arresting officer would persuade someone of ‘reasonable caution’ that the person to be arrested has committed a crime”; when the known facts  “would lead a [person] of ordinary care and prudence to believe and conscientiously entertain an honest and strong suspicion that the person is guilty of a crime”; and when a “prudent person would have concluded that there was a fair probability that the individual had committed a crime.” There are other definitions out there, too; this is just a sampling.

I was a little curious about whether the “fair probability” language has become ever-increasingly prevalent since its unveiling by the United States Supreme Court  in Illinois v. Gates, decided in 1983.  “Fair probability” certainly seems to appear more frequently in recent decisions; do the numbers bear this out?

To find out, I ran another series of searches in the Westlaw ALLCASES database. This time, for each of the years 1980, 1985, 1990, 1995, 2000, 2005, and 2010, I charted the number of cases that featured each of the following phrases, each of which represents the core of a different “probable cause” definition (I have included the Boolean operators that I used in these searches, as well): 1. “fair probability” /p “probable cause”; 2. “honest and strong suspicion” /p “probable cause”; 3. (“reasonable grounds to believe” “reasonable person to believe” “reasonable officer to believe”) /p “probable cause”; 4. “prudent and cautious” /p “probable cause”; 5. “probable cause” /p “reasonable caution.”

I appreciate that there is some mush in the data, since these search terms certainly capture some cases in which the identified descriptive language wasn’t being used to define “probable cause.” But my quick review of the text of the search results suggests that the numbers below appear directionally correct, at least, with the largest number of “junk” results involving the last of these phrasings.

As for these results, the number of cases containing the language above were as follows (with the figures for each year being presented in the same order as the phrasing was listed above):

1980: 0  (FP) / 15 (H+S) / 66 (RG/RP/RO) / 8 (P+C) / 130 (RC)

1985: 113 / 9 / 90 / 10 / 122

1990: 173 / 4 / 116 / 3 / 143

1995: 206 / 7 / 117 / 10 / 182

2000: 246 /10 / 176 / 7 / 245

2005: 470 /33 / 261 / 18 / 315

2010: 747 / 35 / 334 / 16 / 535

So, it looks like the Gates language caught on pretty quickly, battled the previously prevailing “person of reasonable caution” formulation to a draw for a time, and recently pulled ahead–but even today, it only leads, and does not dominate, the field.

I’ve always (OK, only since I started practicing) thought that the “fair probability” phrasing suggests a less stringent standard than the “reasonable caution” language does; but the amorphous nature of probable cause makes it difficult to know what, if any effect, the growing popularity of “fair probability”–if the numbers above are to be believed–has had on mine-run probable cause determinations.

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