The source of this text seems to be anonymous, but the experiment actually occurred. It's from the experiments of Harry Harlow and his associates at the Primate Laboratory of the University of Wisconsin are described in the textbook Principles of General Psychology (1980 John Wiley and Sons). I did a little editing.

A group of scientists placed 5 monkeys in a cage and in the middle, a ladder with bananas on the top.

Every time a monkey went up the ladder, the scientists soaked the rest of the monkeys with cold water.

After a while, every time a monkey went up the ladder, the others beat up the one on the ladder.

After some time, no monkey dared to go up the ladder regardless of the temptation.

Scientists then decided to substitute one of the monkeys. The first thing this new monkey did was to go up the ladder. Immediately the other monkeys beat him up.

After several beatings, the new member learned not to climb the ladder even though he never knew why.

A second new monkey was substituted and the same occurred. The first new monkey participated in the beating of the second monkey. A third monkey was changed and the same thing was repeated (the beating). The fourth was substituted and the beating was repeated.

Finally the fifth monkey was replaced.

What was left was a group of five monkeys who even though they never received a cold shower, continued to beat up any monkey who attempted to climb the ladder.

If it was possible to ask the monkeys why they beat up all those who attempted to go up the ladder, what would be their answer? Maybe, "I don't know. That's how things are done around here."

Sound familiar?