A very effective learning that has come out of many fields of research is that we function well in small groups, specifically groups that range from five to seven people in total. Scrum allows for a slight expansion of that range up to eleven people. This allows for enough people to be together to discuss issues and solve complex software problems with a diversity of skills and experience. As well as avoiding the problem of having too many people that the lines of communication become overwhelming. If the Scrum Team is only four people, then that means that you only have two people doing the tasks in the Sprint Backlog (since the others are the ScrumMaster and the Product Owner). If the team is thirteen or more people then trying to discuss issues, having a focused Daily Scrum meeting, and even building an effective Scrum Team becomes that much harder. The larger the team, the longer it takes to get to a high-performance state.