D: The desire to understand the workings of, or to predict the outcome of something without tangible gain.

Clearly this both requires, and enhances, the subject's rationality. There is no immediate gain, but there is almost certainly a long-term gain - greater ability to predict events and hence to manipulate them in the future.

It seems to be a characteristic unique to humanity - it is unlikely that other animals have it. It is also obvious that humans have it in different quantities. Some will spend their entire lives trying to understand the world around them - others will just sit and watch television.

Of course someone sitting and watching television may be undergoing the same thing at a different level. Trying to understand the social ramifications of a the serial infidelities of a favorite soap-opera star may be an enormous mental challenge for many, and may help them understand their own lives a bit better. Who knows?

See