While talking about the growth of VLTs, Nelson says the following:

I’m always fascinated when people get addicted to virtual things, which often times seem to boil down to a game that involves risk and intermittent reward — where does this tendency come from?

There is a large class of human behaviours which fall under this structure. Consider an action like checking e-mail throughout the day. There is a risk that your checking of e-mail will return nothing (except your disrupted focus), while there is an intermittent reward of getting an interesting or exciting e-mail. Finding food for many animals might be a similar sort of situation. I would guess that the tendency is probably rooted deeply in human (and other animals’) psychology, and serves very useful functions as well as not-so-useful ones.

Each time the fish strike the seabed, for instance, they push off with their fins and expel water from tiny gill openings to jet themselves forward. That, and an off-centered tail, causes them to bounce around in a bizarre, chaotic manner.

Here.