The question I raise in this paper is why measurement systems such as PISA have gained so much power in contemporary education policy and practice. I explore this question from the bottom up by asking what might contribute to the ways in which people invest in systems such as PISA, that is, what are the beliefs, assumptions and desires that lead people to actively lending support to the global education measurement industry or fall for its seduction. I discuss three aspects (...) of what, in the paper, I refer to as the ‘social psychology’ of this dynamics, highlighting the seductive nature of numbers, measurement and comparison, the persistence of technological expectations about education and its workings and the reference to social justice as a key motivator for wanting to know how systems work and perform. I raise critical questions with regard to each of these aspects and, through this, suggest ways towards a more grown-up response to the difficult question of providing good education for everyone, rather.. (shrink)