RESUMO
In order to examine further the role of cultural and environmental factors in human manual preference, two surveys were undertaken in students from Ivory Coast and Sudan. In the first study (Abidjan, Ivory Coast) 382 secondary students, ages 12 to 22, answered a 20-item manual preference questionnaire. The observed frequency of left-hand preference was 7.9%, with very low left-hand use among the 18-22 age group (1%) and high among the 12-15 age group (14%). In the second study (Khartoum, Sudan) 759 undergraduates, ages 18 to 33, answered a 25-item questionnaire. The observed frequency of left manual preference was 5%. Subjects were also asked to indicate any pressure to change hand for writing, eating, or other manual activities and, in the second study, any upper limb injury which temporarily rendered the subject unable to use his (her) preferred hand. Report of an upper limb injury in the past was related to mixed (or inconsistent) hand preference. In both studies, the target activity against left-hand use was eating. These results show that cultural and environmental factors could change "natural" hand preference in three ways: (i) by changing the hand used for only one activity (e.g., eating), with no change for other familiar unimanual activities; (ii) by reducing the degree of hand preference; (iii) by changing the overall preferred hand, generally reducing the prevalence of left-handedness. The design of handedness studies should allow these possibilities to be distinguished.
