Is this object balanced or unbalanced? Judgments are on the safe side.

Do we perceive correctly whether a 2-D object is balanced or unbalanced? What would be the cause of biased equilibrium judgments? In two psychometric studies, we varied independently the characteristics of the objects and the equilibrium states. First, we observed that observers were excessively sensitive to the eccentricity of the object top. Second, we found that the subjective transition from "staying upright" to "falling" occurred before the object would have lost balance according to physical laws. To determine the role of naïve physics beliefs, we tested the effect of physical knowledge. Indeed, participants gave much less erroneous fall responses, once they were acquainted with the physical law. However, a slight bias remained that could not be attributed to an error in locating the position of the center of mass, as fall judgments occurred even when participants judged that the center of mass was above the object base. That is, participants seemed to act according to the principle that it is safer to judge that an object will fall though it would not, rather than making the opposite error.