When I was younger, during summer time, my mom used to get Popsicles for my brother, my sister, and me. She would get a box for us to share, and it had to last us a week (until she went shopping again and could get another box).

Even though my siblings and I knew the box had to last until my mom went shopping again, we usually ran out of Popsicles before the end of the week. Because we each wanted to make sure we got plenty of Popsicles, our conflicting self-interests interfered with our common interest in making the Popsicles last a week.

Whenever one of us was eating a Popsicle and the other two saw the one, that made the other two want to go get a Popsicle too (just to make sure that one person didn’t get more than his fair share, and that I didn’t get less than my fair share.)

There was also the fear that my brother or sister was sneaking an extra Popsicle when I wasn’t around. So I was tempted to sneak an extra one too (just to keep it even).

If my mom had established a regulatory system to give each of her three children a set number of Popsicles, maybe we each could have decided whether to make our Popsicles last the whole week or not. My sister might choose to eat her Popsicles within three days, but I could make mine last all week. However, it would still be fair because we would each get the same number.

By dividing up the Popsicles at the beginning of the week, my mom would have overcome the conflict between our self-interests to get even and our common interests to make the Popsicles last longer. Then, we would bear the full costs of eating a Popsicle now and gain the full reward of saving one until later because the Popsicles were our own private property. As long as the Popsicles were kept as common property, however, we each gained the full reward of eating a Popsicle now, but only had to bear a fraction of the cost of not having one later, so we were motivated to deplete our resources quickly (before the other siblings did).

The depletion of our Popsicles is nothing other than a personal example of the tragedy of the commons. Like the rapid depletion of fishery resources, both could probably be prevented by privatizing common-pool resources. But because of limited enforcement resources, it is probably difficult to have a "mom" who can exercise real authority over national governments to control fishing.