There is a pervasive assumption in the twin and language development literatures that twins are both delayed in language development and more prone to language disorders. In this article, I review and analyze a collection of recent and historic papers which address whether language development in twins is fundamentally different from that of singletons.
To preview, the literature indeed suggests that twins are more prone to language delays and disorders, due to several biological and social factors to be reviewed below. However, many of these factors can (and do) affect singletons - twins are just more vulnerable. Furthermore, most studies show twins catching up to their singleton peers on standardized language tests during early childhood.
I conclude that while twins are more likely to experience language delays and disorders, the risk factors they face are not unique. While twins are at higher risk for language problems, they should not be considered a fundamentally different population.
The early years: 1930s
The pervasive assumption that twins' language development is inferior originated with two papers from the 1930's, Day (1932) and Davis (1937). These papers remain two of the largest studies to date, regarding language and twins.
Day studied 80 pairs of twins and 140 singletons. The children were aged 1.5 to 5.5 years. There were 20 pairs of twins at each of 2, 3, 4, and 5 years of age. 50 spontaneous utterances were recorded from each child while playing with the experimenter's toys. On several gross measures of language complexity (such as sentence length and number of different grammatical categories in a sentence), twins were found to be as much as two years behind singletons by the age of five.
Davis carried the same methodology to twins aged 5-9 and found that on structural measures, twins caught up with singletons on average, but were still more likely to have articulation problems.
There were several methods-based ''flaws'' in the two above mentioned studies, and I am not the first to point them out. Among them, no effort was made to exclude twins who had language, speech, or hearing pathology. Since these disorders are more common in twins, it is possible some of these disorders were present at the time of test, yet the subjects' data was included in the sample, lowering scores and contaminating the results. Second, no information was reported on birth weight or time of gestation of the twins, both of which may be important factors in language delay. Third, birth order was not considered as a possible factor. Birth order is now known to be associated with language disturbances (Matheny & Bruggeman 1972). Fourth, twins in this study were not always observed independently (i.e., separately) and the fact that two children of the same age were competing for the attention of the experimenter may have led to shorter sentences and degraded language samples. Fifth, the twins were averaged as a group and then compared to singletons. However, since each twin's data is not independent of his/her co-twin's, it is not acceptable to average the results and then compare them to singletons as a group.
Twins and Language Development: An OverviewTwins and Language Development: An Overview