Does the science bonus of multiple scientists stack?

It appears to be the best scientist that is used.

To test this, I made a copy of a save file in which I had 3 kerbals that were scientists, each of different levels. In the first copy of the save file, I made a craft with 3 spaces in the cockpit, and put those 3 scientists in, and proceeded to do experiments around the KSC. In the other save, I took the same craft, only putting the scientist in it who was the highest level, and did the same experiments. The science gains were the same, so it would seem that is how Kerbal Space Program figures the bonus. It could possibly have been a fluke, it would be best to try to get some more data points, I'll do some more testing tonight or tomorrow to be sure. After further testing, I have confirmed this.