We could carry on. Would the effect be the same if we tried it in Minneapolis? In Lagos? In Kuala Lumpur, Reykjavik or Alice Springs? Most psychology studies are carried out on urban, affluent, students of the western world – a culturally unusual group, if you take a global or historical perspective. All the subjects studied were “social drinkers”, presumably with some learnt associations about curved and straight glasses. Maybe the Brits had learnt that expensive beer came in curved glasses. If this is the case, the result might be true for everyone who has a history of drinking from straight glasses in the UK, but not for other cultures where alcohol isn't drunk like that.
Little things, big effect
Software entrepreur Jim Manzi calls the rate at which small changes can have surprising effects on outcomes, and the consequent difficulty in drawing general conclusions, “causal density”. It’s because human psychology and social life is so causally dense that we can't simply take straight reports that X affects Y and apply them across the board. But there are hundreds of these relationships reported all the time from the annals of psychology: glass shape affects drinking time, taller men are better paid, holding a hot drink makes you like someone, and so on. Surface effects like these are vulnerable to small changes in circumstances that might remove, or even reverse, the effect you're relying on.
Psychology researchers know all these arguments, and that's why they're cautious about drawing simple conclusions from single studies. The challenge of psychology is to track down those results that actually do generalise across different situations.
The way to do this is to report findings that are about theories, not just about effects. The Bristol researchers show the way in their paper: as well as testing drinking speed, they relate it to people's ability to estimate how full a glass is. They could have just measured drinking speed, but they knew they had to relate it to a theory about what people really believed to come up with a strong conclusion.
If we can find the right principles that affect people's actions, then we can draw conclusions that cut across situations. Unless we know the reasons why someone does something, we'll be tricked time and time again when we try to infer from what they do.