According to an academic at the University of Toronto, intelligence and rationality are different things, which is why apparently bright people can be very stupid at times.

Professor Stanovich says we are 'cognitive misers' who avoid thinking too much, which makes sense from an evolutionary point of view -

Thinking is time-consuming, resource intensive and sometimes counterproductive. If the problem at hand is avoiding the charging sabre-toothed tiger, you don’t want to spend more than a split second deciding whether to jump into the river or climb a tree.
For example, take this little puzzle, which apparently more than 80% of people get wrong:

Jack is looking at Anne, but Anne is looking at George. Jack is married but George is not. Is a married person looking at an unmarried person?


Yes/No/Cannot be determined
If you chose 'Cannot be determined' (I did), then you were one of them. Of course, knowing the context will make you work harder at it, but the point is that:

unprompted, they won’t bring their full mental faculties to bear on the problem.
Full article here.