Whoever came up with this claim was probably thinking of the Coriolis effect, which explains the apparent deflection of objects moving in the sky.
The Earth's equator rotates eastward at a much faster speed than its higher and lower latitudes – in fact, the further you move from the equator, the slower you are rotating around on Earth's daily 360-degree spin. This means that an object suspended in the air, such as a plane, will appear to move differently depending on where you're standing. An object traveling North will appear to travel to the right, and one going South will appear to move to the left. In reality, the suspended object is moving in a straight line, but it's the Earth's rotation that causes its apparent bend.
The effect explains why winds in the North rotate counterclockwise, and those in the South move clockwise. People who claim the effect has an impact on how your household water drains are actually partially right and partially wrong. While the effect is not powerful enough to affect the draining direction in toilets, sinks, or bathtubs, one MIT professor has proven that it's possible.
In 1962, Ascher Shapiro filled a small circular tank with water and let it sit for 24 hours to get rid of any movement that could affect rotation. He also covered the tank to protect the water air currents and kept it at a constant temperature. When all of these conditions were met, the water drained in a counterclockwise vortex every time.
But a number of things can affect the way your water drains, including the structure of the drain, the surface of the sink, or where you might be pouring it.
There are plenty of tourists who have fallen prey to this "scientific wonder." Con men near the equator have been known to pour water in basins on one side of the equator and then the other, demonstrating how the different hemispheres affect the water's rotation. But these men can influence the way the water drains with sly tricks or movements. "Bad Astronomy" blogger Phil Plait points out that in one video, the man pours water on a different side of the basin each time, causing the water to flow differently in each hemisphere.
