The water-level task is an experiment in developmental and cognitive psychology[1][2][3][4][5] developed by Jean Piaget and Bärbel Inhelder.
It was first described in their book The Child's Conception of Space, published in French in 1948, with an English translation appearing in 1956.
[1][7] They described a series of stages children pass through in their understanding, corresponding to different modes of performance on the water-level test, before mastering it around the age of nine.
[1] It is difficult to give the precise fraction of men and women that fail the water-level task, since this is sensitive to the methodological details of how the task is presented and scored, but the finding that men perform at a higher level has been robustly confirmed.
[8] The difference in performance between men and women has been estimated, in terms of Cohen's d, to be between 0.44–0.66 (i.e. between 0.44 and 0.66 standard deviations).