The Development of Language in Twins

AN'S curiosity in the unusual has led him into many fields of adventure. Twins, constituting about one per cent of the births according to Davenport (3) are both so rare and so different that science has for a long time been very curious about their origin, their resemblances and their differences. The dispute as to the relative importance and effects of heredity and environment in moulding human nature dates back to the ancient Greek philosophers and scientific workers always have felt that twins presented a special opportunity for making such studies. Newman's (16) investigations on the star fish and the Texas armadilla as well as his more recent studies of twins, point unquestionably to the fact that there are 2 types of twins. The identical or monozygotic twins arise from the splitting of a single zygote while the fraternal or dizygotic twins arise from the simultaneous development of two zygotes. The psychological studies of twins beginning with that of Galton (4) in 1883 also give evidence of the two types of twins, in that they show a greater degree of resemblance in both physical and mental traits in identical twins than in fraternal twins. Thorndike (19) in 1905 disputed this fact. The later investigations of Merriman (14), Lauterbach (12), Gesell, (5, 6), Muller (15) and Wingfield (22) however have all supported the theory of the monozygotic origin of identical twins. This investigation, though undertaken primarily to compare language development in twins with that in single children, also obtained sufficient information in regard to the twins studied to determine fairly accurately whether or not they were identical or fraternal twins. With these data at hand, the three types of twins, identical, like-sex fraternal and unlike-sex fraternal were compared as to resem1 From the Institute of Child Welfare, the University of Minnesota. The Development of Language in Twins, Part I, A Comparison of Twins and Single Children appeared in Child Development (Vol. 3, No. 3). It reported the language development of 80 pairs of 2 to 5 year old twins and compared their development with that of the single children reported by D. A. McCarthy (13) in The Language Development of the Preschool Child. McCarthy's method and technique were repeated throughout. The tables of Part 1 and 2 are numbered con-