Originally Posted by far_beyond_sane

1) Demand characteristics/the placebo effect - does the environment or design of the experiment suggest, consciously or unconsiously, how people should behave in it?

2) Construction of variables - for example, exactly what is 'verbal reasoning'? Or 'intelligence'? Or any number of other skills, qualities, states of mind etc. that we are quite comfortable with as concepts but have great difficulty in quantifying?

3) Correlative-causative fallacy - a fancy-arse way of saying that things that co-occur may do so because of a third unrelated factor.