1. Newness is not a benefit

So newness is a liability?

Absolutely. The research confirming that is incontrovertible. Consider crews flying commercial airplanes. The National Transportation Safety Board found that 73% of the incidents in its database occurred on a crew’s first day of flying together, before people had the chance to learn through experience how best to operate as a team—and 44% of those took place on a crew’s very first flight. Also, a NASA study found that fatigued crews who had a history of working together made about half as many errors as crews composed of rested pilots who had not flown together before.

Implication: Don't leave team set-up to chance

2. Size Matters

Another fallacy is that bigger teams are better than small ones because they have more resources to draw upon. A colleague and I once did some research showing that as a team gets bigger, the number of links that need to be managed among members goes up at an accelerating, almost exponential rate. It’s managing the links between members that gets teams into trouble. My rule of thumb is no double digits. In my courses, I never allow teams of more than six students. Big teams usually wind up just wasting everybody’s time. That’s why having a huge senior leadership team—say, one that includes all the CEO’s direct reports—may be worse than having no team at all.

Implication: Don't allow teams to get bigger than 10

From: D. Coutu HBR May 2009 "Why teams don't work"