Fruit machines (or slot machines as we call them in the states) are very
addicting to people because of the way they are set up to give out money.
Our behavior is based on schedules of reinforcement (reward). That is, all of
our behavior is motivated by its consequences. If you give someone roses, they
give you a kiss. The kiss is a reward and "reinforces" you to give them roses again. This is a "fixed-ratio" schedule. Every time you gives roses, you get
a kiss. With a fruit machine, the schedule of reinforcement is a "variable-
ratio" That is, the ratio of times putting money in to times being rewarded
is random. You never know when you will win, but you know that you CAN win.
It so happens that behavior that is rewarded on a "variable-ratio" schedule is
the most difficult to extinguish. People have a hard time stoping at fruit machines because they figure "maybe THIS time I'll win". If they don't, they still hopr it'll be next time.
The great B.F. Skinner is the man to read if you want more info about this.
Much has been written about the psychology of gambling, but i hope that I've helped somewhat in outlining basic constructs.
