The Simpsons episode "Bart vs. Australia," which involves the oldest Simpson kid getting indicted for fraud in the Commonwealth, starts with a scene in a bathroom. Bart has noticed that the water in the sink always drains in a counterclockwise way; Lisa informs him that, in the Southern Hemisphere, it drains the other way. Bart doesn't believe her. To find out for sure, though, he calls a number in Australia—collect—and … hijinks ensure.
The idea that water rotates differently in the different hemispheres is a long-standing one. Long before seeing that Simpsons episode, I'd heard it, and assumed it to be true. It sounds true. Just like Lisa explains to Bart, the Earth is subject to Coriolis forces, which determine how moving objects are deflected off the inside of rotating objects—wheels, circular containers, those kinds of things. Applied to Earth's rotating sphere, the Coriolis effect accounts in part for why, say, hurricanes and cyclones rotate the way they do. (The storms rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern.)
In 2001, Scientific American asked Brad Hanson, a staff geologist with the Louisiana Geological Survey, to explain (in theory) how those forces could affect drains. He replied that Coriolis effects:
can be visualized if you imagine putting a pan of water on a turntable and then spinning the turntable in a counterclockwise direction, the direction in which the earth rotates as seen from above the north pole. The water on the bottom of the pan will be dragged counterclockwise direction slightly faster than the water at the surface, giving the water an apparent clockwise spin in the pan. But if you were to look at the water in the pan from below, corresponding to seeing it from the south pole, it would appear to be spinning in a counterclockwise direction. Likewise, the rotation of the earth gives rise to an effect that tends to accelerate draining water in a clockwise direction in the Northern hemisphere and counterclockwise in the Southern.
Here's the catch, though. Coriolis forces are best observed at a large scale; toilet water, in so many ways, is small-scale. As Robert Ehrlich, a physicist at George Mason University, explains it: "The effects of the rotation of the earth are, of course, much more pronounced when the circulation covers a larger area than would occur inside your bathtub."
In your tub, such factors as any small asymmetry of the shape of the drain will determine which direction the circulation occurs. Even in a tub having a perfectly symmetric drain, the circulation direction will be primarily influenced by any residual currents in the bathtub left over from the time when it was filled. It can take more than a day for such residual currents to subside completely. If all extraneous influences (including air currents) can be reduced below a certain level, one apparently can observe that drains do consistently drain in different directions in the two hemispheres.
So what does account for the varying ways that toilets flush and drains drain? The design of the objects that do the flushing and the draining, for the most part. (Not to mention things like buildup and debris, which can affect the shape water takes as it's moving through pipes.) Here's Neil deGrasse Tyson—he who plucked Pluto from the status of "planet"—busting another misconception:
Toilet Bowls drain however they're designed to circulate water. It’s irrelevant whether you live above or below the equator.— Neil deGrasse Tyson (@neiltyson) September 19, 2014
There you have it. Another myth, going down the ... well, you know.