Take the classic visual illusion called the Müller-Lyer illusion - the one with two lines of equal length, where line "a" appears shorter than line "b" simply because of the way the arrows on their ends are oriented (see diagram). Way back in the early 1960s, psychologist Marshall Segall at the University of Iowa in Iowa City led a team testing the susceptibility of people from different cultures to this illusion. They manipulated the length of the two lines until observers judged that they were the same, then recorded this point of subjective equality (PSE) - the extent to which "a" had to be made longer than "b" for the two to be judged equal.
PSE is a measure of the strength of the illusion, and Segall found that students in Evanston, Illinois, were by far the most affected, requiring "a" to be almost 20 per cent longer than "b" before they judged the two equal. The PSE for the aboriginal San people of the Kalahari desert, at the other end of the spectrum, was close to zero. The illusion wasn't even an illusion for them.
This finding is not as trivial as it might at first appear. It implies that a fundamental aspect of perception, which had till then been assumed to be hard-wired and therefore common to everyone, is actually shaped during our development by some aspect of the culture in which we live. Though we are far from understanding this effect, Segall and colleagues suggested a possible explanation: people who grow up among carpentered corners - WEIRD people, for example - might be tuned by the geometry of their world to render them more susceptible to the illusion.
So, if you are WEIRD, you perceive the world oddly. You also have a funny way of describing it. English, the lingua franca of the WEIRD world, relies on a system for locating objects that is egocentric or relative to self, as do other Indo-European languages. So an English speaker might say: "The police officer is to the left of my car". It was assumed for a long time that this was true of all languages - but then exceptions began to crop up. These usually entailed an allocentric frame of reference, describing the location of objects relative to points outside the self, such as the points of the compass ("The police officer is west of the car") or some other object ("The police officer is between the car and the kerb").