Supersize me!

If you go to 7-11 to buy a slurpee, as I often do, you're faced with a decision. They come in three sizes: small, medium and large. I usually only have slurpees when I'm in Melbourne, I think because I need them more down there, but I had one today in Sydney (hangover, three beers last night) and I was musing about this size issue today.

The small costs $1.70, the medium $2.00 and the large $2.30. When I first started on this downhill spiral of slurpee abuse earlier this year (in an episode I described to my friend Melanie as me sitting in a borrowed car in the parking lot of the 7-11 in Mount Waverley, having a slurpee, banging my head againt the steering wheel and weeping) I used to go with what seemed like the value play. The medium's much bigger than the small, the large is, in turn much larger than the medium and it seemed reckless and foolish not to just get the large. It's about twice the size of the small, and not much more money.

But after lots and lots of research (I have a good mental map of the eastern suburbs of Melbourne, with all the 7-11s on it) I've come to a shocking conclusion: the optimal size for a slurpee is small.

"How can this be so?", I can hear you ask. "Have you taken leave of your senses?". And so on. Just calm down. No, the horrible truth is that if you get a big slurpee, when you get about halfway through it you get a bit bored. You reach a point you never imagined existed - the point where you've had enough slurpee. (In the same way that as a 16year old, you can't imagine how, as a new parent, exhausted after sleepness nights, you can be too tired for sex.)

After quite a bit of empirical work, I realised that this point - the point where the marginal utility goes negative - is about where you would be if you'd only had the small. So there's your answer. I know you'll thank me for this later.