Here is a scene you already know. Five students get a group project. In week one, everyone is enthusiastic. In the final week, one person is up at midnight doing almost everything, two people did a bit, and two people did nothing except appear in the presentation.
Now the strange part: all five are decent, hardworking people on their own. Put them in a group, and the group produced less than its best member would have produced alone.
This is not bad luck. It is one of the most reliable findings in all of psychology, and once you can name it, you can fix it.
The rope experiment
In the 1890s a French engineer named Max Ringelmann asked people to pull on a rope as hard as they could — first alone, then in groups. Alone, people gave 100%. In a group of eight, each person pulled at roughly half their solo strength. And here is the unsettling bit: they had no idea they were doing it.
Notice the key phrase: cannot be seen. Nobody decides to slack. The effort just quietly leaks away when nobody can tell who contributed what.