The 2D:4D digit ratio is the quotient between the length of the forefinger and the ring finger and an indirect marker of androgenization in the first trimester of gestation. A lower 2D:4D ratio reflects prenatal exposure to higher levels of testosterone, which results in a low ratio in boys and a high ratio in girls.1 Several studies have demonstrated an association between prenatal testosterone levels and the development of the cerebral hemispheres, inducing the maturation of the right hemisphere or the acceleration of programmed cell death in the left hemisphere.2 These asymmetries in cerebral structure affect various cognitive functions, including language.3 Thus, several studies have demonstrated the important role of testosterone and other prenatal hormones in the presence of sexual dimorphism in the development of spatial skills4 and language,5 especially in the areas of vocabulary and fluency.6

Taking into consideration this association between prenatal androgenization and language development, Geschwind and Galaburda7 hypothesized that an increased prenatal exposure to testosterone would be associated with a higher incidence of language and learning disorders in male compared to female individuals.8 Although several studies support this hypothesis,9,10 the findings of other studies contradict it, both in individuals with and without disorders.7,11–13 Furthermore, the studies that have found evidence of this association could not firmly establish the direction of the effect, that is, whether language problems are associated with a higher or a lower 2D:4D digit ratio, in either the general or the clinical population.14

The contradictory findings of previous studies call for the continued investigation of the association between the degree of androgenization and different language problems as well as the role of nonverbal intelligence in this association. In this sense, it would be key to study this ratio in relation to specific language impairment (SLI), as affected individuals have language problems in the absence of other developmental impairment.15 Furthermore, we know of no studies on this subject in individuals with SLI, and establishing an association between the 2D:4D ratio and SLI would provide a useful biological marker that would contribute to the early diagnosis and treatment of SLI.

Children with SLI have language difficulties in the absence of the causes that usually explain these problems, such as intellectual disability, brain damage, hearing problems or socioemotional problems.15 Their language is mainly characterized by impairments in morphology and syntax, although they may also present problems in expressive vocabulary and speech.16 Neuroimaging studies of individuals with SLI have shown deviation from the normal pattern of volumetric asymmetry of the cerebral hemispheres17 that could be explained by prenatal exposure to high levels of testosterone.3,18,19

In addition, several studies have found an association between SLI and various behavioural disorders,20–22 and it is known that prenatal androgenization increases aggressive behaviour in every species in which this association has been studied, including primates.23 Thus, in children with SLI, increased prenatal exposure could explain the language impairment as well as behavioural problems, which would result from the increase in aggression.

With the ultimate aim of identifying a biological marker of SLI and with the hypothesis that there is a correlation between the 2D:4D digit ratio and language development and adaptive behaviour, we conducted a study with the following objectives: a) to compare the 2D:4D digit ratios in children with SLI versus children with normal language development, b) to describe the behavioural problems associated with the presence of language impairment, and c) to analyze the correlation between the 2D:4D digit ratio and aggressive behaviours in children with SLI.

Our results suggest that it is possible that the language problems of children with SLI are associated with decreased prenatal exposure to testosterone, based on right-hand digit ratio values. We did not find an association with the left-hand digit ratio, which was consistent with previous studies.9,10 Thus, our results contradict the Geschwing-Galaburda hypothesis7 and those of other studies that found a lower 2D:4D digit ratio in individuals (with and without ASD, cognitive impairment or SLI, among others) with poorer language skills (especially in the areas of speech and vocabulary, with no differences in empathizing).9,10,12

In our study, prenatal exposure to lower levels of testosterone was associated with a higher future probability of developing a specific language impairment in the absence of cognitive impairment, especially involving morphology and syntax as opposed to speech and/or vocabulary. Therefore, our results were similar to those reported by Manning et al.14 in that children with language impairment in the absence of cognitive impairment had higher-than-normal digit ratios.

Several previous studies11,13,14 have already evinced the complexity of the association between androgenization and various language and communication problems. For instance, Albores et al.,9 who analyzed the association between the 2D:4D ratio and various measures of language development, did not find a significant correlation between this ratio and language delay. However, they did find a significant correlation between the right-hand 2D:4D digit ratio and speech problems.9 On the other hand, other authors have proposed that the association between prenatal testosterone exposure and language skills may not be linear and may vary between male and female individuals.4 For instance, Burton et al.4 found that men that had been exposed to higher levels of testosterone prenatally (lower 2D:4D ratio) exhibited more severe impairment in language and spatial skills, while women that had been exposed to lower levels of testosterone (higher 2D:4D ratio) exhibited the poorest language and spatial skills. There is also evidence of these complex patterns of association in the clinical population. Thus, Manning et al.14 found a lower 2D:4D digit ratio in children with ASD and cognitive impairment, while children that had ASD without cognitive impairment (or even with above-average intelligence) had greater-than-normal 2D:4D digit ratios.

When it came to behavioural variables, our study also found that, overall, children with SLI have poorer social and leadership skills, more attention problems and a greater tendency to somatise, that is, a greater tendency to internalize problems and poorer adaptive skills, which probably has a negative impact on their social relationships, academic performance and psychological wellbeing. Our results were consistent with those of previous studies.21,22,28,29 For example, Valera-Pozo et al.30 also found significant differences between a group of preadolescents with SLI and another group with normal language development in measures of social skills, leadership and adaptive skills reported by form teachers.

However, based on our findings, boys aged 5–8 years with SLI do not generally tend to be aggressive. Similarly, they show no tendency to exhibit abnormal behaviour or conduct problems. This has also been confirmed by some studies in the literature,20 which have highlighted the absence of aggression as a distinctive behavioural characteristic of children with SLI.

Last of all, our study shows that intrauterine testosterone levels do not seem to have an effect on any behavioural endpoint. However, we did find a correlation between several dimensions of behaviour, the level of language development and the level of intelligence, which suggests that language skills have a greater impact on behaviour than intrauterine testosterone levels. Thus, it appears that behavioural problems in children with SLI are not the result of prenatal androgenization, but rather mediated by their language difficulties. In other words, language problems in children with SLI would result in the development of fewer social relationships and thus on poor development of social skills. This lack of social skills would generate anxiety in anticipation of social interaction, as well as depression, for these children would wish to have relationships but not know how to develop them. In turn, this would lead to internalizing problems (somatization), as difficulties in communication would prevent these children from externalizing these problems. This hypothesis is supported in part by several previous studies with similar characteristics and results.28,30,31

The findings of this study must be interpreted with caution due to a number of limitations: the small sample size, which, while common in this type of study, does limit its conclusions; the fact that we did not have access to a group of girls with SLI and the corresponding control group to compare results with the male sample, and not having collected more detailed information on hormone levels during gestation. Further research on this subject taking into account these limitations would be desirable.

Based on our findings, prenatal exposure to lower levels of testosterone (higher 2D:4D ratio) is associated with a higher risk of developing a specific language impairment in boys. The intrauterine testosterone level does not seem to be associated with the different aspects of social behaviour that are altered in children with SLI (attention problems, depression, somatization, social skills, leadership, internalizing problems and adaptive skills), aspects that, on the other hand, could be explained by the language skills and intelligence level of children with SLI.

The authors have no conflicts of interest to declare.