Judges are more lenient on defendants' birthdays

Are you nice to people on their birthdays? Probably you are. Most people are. It's a social norm. It turns out that this social norm also extends to judges' decisions about sentencing defendants, as shown in this recent article by Daniel Chen and Arnaud Philippe (both University of Toulouse Capitole), published in the Journal of Economic Behavior and Organization (ungated version here).

Chen and Philippe first look at judicial decisions in France, using data on 4.2 million sentencing decisions over the period from 2003 to 2014. Importantly, in this context:

Judges in correctional courts (for misdemeanor) have no control over their schedule. For each case, when the investigations are finished, the prosecutor in charge chooses the type of procedure (accelerated/normal) and, based on this, picks the next session of the relevant type. The weekly schedule of the sessions is fixed and decided at the beginning of the year by the head of the court with little discretion to select trial dates on defendant birthdays.

So, whether a defendant is sentenced on their birthday or not is effectively random (and Chen and Philippe establish this with some statistical checks in the paper), and which judge the case is assigned to is unrelated to whether it is a defendant's birthday or not. Are judges more lenient on defendants' birthdays? The results are neatly summarised in Figure 2 from the paper:

Notice that the average sentence is substantially lower on a defendant's birthday (the red column) compared to days on either side of their birthday (the blue columns). Statistically:

Results are consistent and indicate that sentences are reduced by roughly four days... On average, sentences are up to 6.2% shorter on defendant birthdays.

So, judges in France are more lenient on defendants' birthdays. Chen and Philippe then turn their attention to the US, where judges have a bit less discretion. As they explain:

Cases are randomly assigned to a single judge. The United States Sentencing Commission (USSC) produces sentencing guidelines for federal judges. The judges are given a guideline range for the criminal sentence that is based on the severity of the crime and the defendant’s criminal history. Due to these guidelines, the largest factor determining sentence range is the criminal charges brought to the judge by the prosecutor. Therefore, we expect the effect of a birthday to be more limited than in France, where judges have more discretion.

Because of the sentencing guidelines, judges have little discretion over the length of the sentence (measured in months), but can vary the number of additional days in the sentence (so, for example, a sentence of 15 months and six days is more lenient than a sentence of 15 months and 20 days). Chen and Philippe therefore focus on differences in the day component of the sentence for US defendants. Their US data is based on over 600,000 sentencing decisions between 1991 and 2003. And their results look very similar to those for France, and are best summarised in Figure 4 from the paper:

Notice again that the red column is much smaller than the blue columns. Statistically:

...the number of days in a federal sentence declines on defendant birthdays, but not on the days before or after birthdays... We find that judges assign 0.13 fewer days if the decision occurs on the defendant’s birthday, all else equal. The effect is about one-third of the average number of days (0.36). We also see no impact on the days before or after the birthday.

So, judges in the US are more lenient on defendants' birthdays. Interestingly, with the US data Chen and Philippe dig a little bit deeper into judicial thinking, since within that data they know which sentences were given by which judges. They also have a dataset of their written judicial decisions. Using those data:

We measure judges’ use of deterrence language and consider it as a proxy for “economic reasoning”... We find that judges below-median in economic thinking are affected by birthdays, decreasing the day component by 0.17, while those above-median in economic thinking are essentially unaffected by birthdays.

Now, if we interpret Chen and Philippe's measure of 'economic reasoning' as a measure of whether judges make decisions in a rational way (in the economic sense), then it appears that judges who are more rational are less affected by the social norm of favouritism on birthdays. That is what we might expect from rational decision-making, which should be based on the costs and benefits of the alternatives (and this applies in sentencing, just as it does in other decision contexts).

Now, Chen and Philippe bury a lot of the detail on this analysis into Appendix C to the paper, but to some extent this is the most interesting of their results. In fact, it would be really interesting to explore this further. Judges' decisions have previously been shown to be affected by whether the decision is made before or after lunch, or affected by weather conditions. It would be interesting to see whether judges who are more rational are less affected by those irrelevant factors as well. There is definitely an opportunity for future research in this area.