B.F. Skinner.

This should prove interesting...

So, you have a pigeon and a cage. Skinner proved you may easily train the pigeon to peck a  specific number of times in order to get food/ (aka reinforcement).

He started off by delivering a reinforcer per peck. Later on, tried delivering a reinforcer every other peck. He noticed that pretty soon the pigeon would only peck in groups of two. The same happens if you change the schedule to three pecks etc. This is called a partial reinforcement FR3 - fixed ratio 3 and so on. Skinner also managed to get high FRs, such as FR20, FR50. 

A more interesting schedule of reinforcement is the VR - variable ratio: The animal is started off on continuous reinforcement for a while (~FR1). After that you build a schedule with a slightly bigger FR value and do this till you get to ~FR15, FR20.

You now move to a VR2. VR2 means every second response gets reinforcement, but you never know trial to trial which response will. That's determined randomly. You now stretch out the VR2 to a VR5, VR10, VR20.

Virtually for the rest of that animal's life (even if he doesn't receive reinforcement at all) it will peck ad infinitum. Skinner refers to behavior shaped that way as gambling behavior.