An interesting question posed by Richard Thaler over at the Edge:

The flat earth and geocentric world are examples of wrong scientific beliefs that were held for long periods. Can you name your favorite example and for extra credit why it was believed to be true?

Some are interesting:

In the early days of the field of Artificial Intelligence, researchers thought that it would not be terribly difficult to implement a vision recognition or language understanding program.
…
The importance of these misperceptions is the underestimation of the complexity of the brain.

and some are utterly superfluous.

That parrots were not only stupid, but also could never learn to do anything more than mimic human speech.

It was believed to be true because the training techniques initially used in laboratories were not appropriate for teaching heterospecific communication.

Eyeroll.

My answer would be: that superstition and ‘spirituality’ are suitable ways to answer questions about our world and that ‘having faith’ represents some kind of virtue. Spirituality, superstition, and faith have never contributed one thing to human knowledge because the answers they give come from intuitions and preconceptions, not from real data about the world. Yet many otherwise smart and educated people are still unable to relinquish such faulty methods.

My favourite comment comes from Charles Simonyi.

I think we are all too fast to label old theories “wrong” and with this we weaken the science of today — people say — with some justification from the facts as given to them — that since the old “right” is now “wrong” the “right” of today might be also tainted. I do not believe this — today’s “right” is just fine, because yesterday’s “wrong” was also much more nuanced “more right” that we are often led to believe.