The symmetry hypothesis and the perpendicular error: Evidence from discrimination and copying tasks

The interpretation by Breniner & Taylor (1982) that the tendency by children to increase oblique line angle size when copying line drawings (the ‘perpendicular error’) is one of ‘symmetry’ is examined using a discrimination and a copying task. Using single right-angled triangles as stimuli, errors by older children in a discrimination task are found to be in the direction of ‘symmetry’. Evidence from a copying task using similar stimuli favours a simpler interpretation of oblique line errors in terms of a ‘vertical effect’. Strategies for copying remaining lines in such figures are analysed in terms of directional ‘rules’, the usage of which increases with subject age.