We all know the effect and Berega (along with the 7 other runners who broke 13) just demonstrated it. We also feel how much easier workouts (at similar paces) are when in a group.
Intuitively we all know it is psychological, but it would be interesting to see just what percentage of an athlete's personal best requires that psychological push. What is the absolute best Berega would do on a solo time trial? Would he do better in a solo time trial than if he ran with a bunch of 5k'ers paced at 13:30? Are the pacers the only reason Kipchoge got close to breaking 2 and the gigantic clock car and Formula One track had very little to do with it? Just curious if it has been formally looked at.