Directional biases in adults' performance on a multiple-choice water-level task

Performance data on the water-level task were collected on a multiple-choice test requiring 417 adult subjects to choose the one item that represented the waterline of still water in a tilted vessel. Except for the item showing the waterline horizontal, waterlines were drawn oriented +/- 8 degrees, +/- 16 degrees, +/- 24 degrees, and +/- 32 degrees referenced to the horizontal. Analyses of errors from 284 subjects who did not achieve a perfect test score showed that 73% of all errors were made on negatively signed items. Further, among subjects who erred on all trials (n = 128), the majority were remarkably systematic in making their errors, consistently (six out of six trials) choosing items with a negative sign (56%) or items with a positive sign (13%). The predominant negative directional bias suggests that most adult subjects who completely lack an adequate understanding of the invariant horizontality of water were basing their choices on the belief that water tends to climb obliquely towards the brim of the tilted vessel.