Is the Supreme Court more divided than ever?
Today the Supreme Court ruled unanimously (9-0) that police cannot install a GPS device in a car to track a suspect without first obtaining a warrant to do so. I’m always kind of satisfied when the Court has unanimous decisions; it makes me feel like the issue is well-settled. No disagreement. Done.
Yet it has seemed in recent years that so many decisions are 5-4 splits. That the Supreme Court has become politicized in a way unlike ever before. But is this really true? Is the High Court any more ‘divided’ than it has been in the recent past? Let’s look at some data.
I downloaded the Supreme Court case record from 1946-2010 from the Supreme Court Database and plotted the proportion of cases decided by majorities of different sizes, by year. If the court were ‘more divided’ now than before, we would expect more cases won by a majority of 5 than in the past. That is shown in purple, and unanimous decisions are in yellow.
Some interesting trends come from this. You can see that in the 90s there seemed to be quite a lot of cases decided unanimously. And since around 1990 or so there seem to be fewer cases decided by a majority of 6 (in red) than there had been before. And it does appear that the past 10 years or so have had slightly more decisions with a majority of 5 than the average for the past 60 years.
It it important to note that I haven’t ‘proved’ any of these conclusions in any sense. But by exploring the data in this way we can find patterns that help us to understand where to look more closely. Then we can perform an explicit statistical test. What patterns can you find from exploring this data?
UPDATE: Rethinking things, a ‘divided’ court should still be expected to decide unanimously sometimes: some cases are just very clear. An ‘undivided’ court should be one in which cases are divided by margins other than 5 or 9. If we plot the proportion of cases won by a majority of 5 or 9 as a function of time we get this plot. Here, there is a really clear trend: over the past 30 years the court has been increasingly either voting unanimously or narrowly divided.