Ibbotson A, Bryant P E
Perception. 1976;5(3):319-26. doi: 10.1068/p050319.
It has been known for some time that children have particular difficulty drawing lines that are not perpendicular. But this difficulty has not been studied systematically. By systematically varying, in three experiments, the baseline, the response, and the kind of figures to be copied, we showed that the tendency to draw angles as more perpendicular than they actually are is a general one which occurs as much with abstract as with meaningful material and with radically different responses. We also discovered an unexpected effect, the vertical effect, which takes the form of the error occurring much less when a vertical baseline is involved than with other baselines.
