III. Euidence for inheritance of language potential
The inheritance of behavioral traits …Last paragraph on P.258 missing? … in apparently different environments.
We recognize these difficulties. If the supposition of inheritance of language potential were based on merely one type of evidence, we could not make a strong case for it. However We are fortunate in that all types of evidence usually adduced for inheritance in man, also support the thesis that genetic transmission plays a role in our ability to speak.
Scientific investigations into genetics of language and its disorders have been carried out since the beginning of the century, (Gutzmann, 1916; Orton, 1930; and many other cited by Luchsinger, 1959). In more recent years the familial occurrence of a variety of speech disorders has been described by Huyck (1940), Mussafia (1960), and pfaendler (1960). A closely related disorder, congenital dyslexia, a peculiar difficulty with learning to read, also tends to run in families (Drew, 1956, and Hallgren, 1950). On the basis of these studies, a clinical syndrome has been identified called congenital language disability. Consisting of a combination of any of the following deficits: markedly delayed onset of speech (in the presence of otherwise normal developmental milestones), poor articulation often persisting into the teens. Poorly established hand preference or frank leftism, marked reading difficulties. Either complete inability or marked difficulty for acquiring a second language after puberty (Eustis, 1947; Gallagher. 1950). Intelligence is usually not affected.
The familial occurrence of congenital language disability is well documented through a number of published pedigrees (Luchsinger and Arnold. 1959; Arnold, 1961; brewer, 1963) partly reproduced in Fig. 6.8. After a survey of all published cases we may well agree with Brewer’s conclusion that congenital language disability is probably a dominant, sex-influenced trait with at least fair penetrance.
Pedigrees for more specific language and speech disturbances are also available, notably for the occurrence of stuttering (Luchsinger, 1959). Luchsinger (1940) has also studied the familial incidence of the general ease of language-expression. Of particular interest are the extremes of the normal range. Both global language-facilitation and global language-difficulties may be attributed to inherited factors. Transitions from borderline normal to borderline abnormal appear as delayed onset of speech, protracted articulatory difficulties in childhood, congenital expressive disorders, or conversely, hyperfluency resulting in cluttering or pathological talkativeness. In the latter instance, ease of communication and intelligibility may suffer as much as in individuals who actually suffer from the opposite pole of the disorder-scale.
Type and translate the legend of Fig. 6.8 here.
Language in light of evolution and genetics
In a recent summary, Luchsinger (1959) also surveyed the literature on the familial occurrence of word-deafness and speech-sound deafness, of congenital difficulties with acquisition of grammar, and of psychological variables on the heritability of voice qualities.
The role of genetic factors may be further elucidated through comparison of speech development of fraternal and identical twins. A fairly impressive number of studies exists. Aspects of voice in twins have been studied by Schilling (1936), Luchsinger (1944), Gedda, Bianchi, Bianchi, and Neroni (1955), and Gedda, Fiori-Ratti, and Bruno (1960). Speech and language disturbances of twins were investigated by Seemann (1937), Brodnitz (1951),and Luchsinger (1953, 1957, and 1961), and language psychology by Gottschick (1939). The investigations are reported in varying detail and thoroughness which makes pooling of data and statistical computations of level of significance impossible. Only Luchsinger’s work includes blood typing of subjects and complete tabulation of all relevant facts.
Type and translate legend and illustrations of Fig. 6.9 here.
III. Euidence for inheritance of language potential
All studies agree that fraternal twins are much moreprone to differences in language development than identical twins. The proportions shown in Fig. 6.9 are essentially based on Luchsinger’s publications but have been rounded off by comparing his findings with those of Seemann (1937). The diagrams are merely intended to show the trends in rough figures.
The developmental history of identical twins tends to be much more synchronous than that of fraternal twins. Among the former, motor milestones, menarche, change of voice. And growth rate are more likely to occur at the same time than among the latter. The same is reflected in the onset of speech (that is, the age at which the first few words appear, words are joined into phrases. And grammatical mistakes become minimal). It is only among the fraternal twins that difference in onset occurs, whereas identical twins seem to progress simultaneously. It is the general consensus of the investigators that these divergences cannot be simply explained on the grounds of imitation or differential treatment by parents.
If the developmental history as a whole is compared, that is the occurrence of milestones and/or the occurrence and nature of speech and language abnormalities, the difference between identical and fraternal twins is even greater. In this case, however, it is more difficult to refute the hypothesis that fraternal twins who tend to look different will not react to one another as easily as identical ones. But it is also not possible to prove this hypothesis and to disprove also the assumption of genetic differences.