Sex-Specific Differences in Infant Visual Attention and Early Word Production: Potential for Female Protective Factors

Background: Autism Spectrum Disorder (ASD) is among the most highly heritable of all psychiatric disorders, with a large body of research supporting a multifaceted genetic etiology. One well-documented but still poorly understood feature of that etiology is a highly skewed sex ratio, with only one affected female with ASD for approximately every 4 males (Werling & Geschwind, 2013). It has been suggested that there may exist some mechanism of “resilience” in females such that risk factors are less potent. We hypothesize that sex-specific differences in processes of typical development (TD)—i.e. female precociousness in acquisition of communicative milestones—could serve as such a protective factor and could support female resilience to ASD. Previous research from our laboratory suggested sex-specific differences in visual attention to the mouth in the first 18 months of life, temporally and developmentally aligned with language acquisition.

Objectives: We studied sex-specific differences in both visual attention to the mouth and in language acquisition, from 2 until 40 months of life, in order to better understand their potential role as protective factors for females at risk for ASD.

Methods: Eye-tracking data were collected in TD infants longitudinally from 2 until 40 months of life (146 male, 131 female). The MacArthur-Bates Communicative Development Inventory (CDI) was administered longitudinally at 9, 12, 18, and 36 months, assessing vocabulary acquisition. Z-scores were calculated from raw CDI scores.

Results: A marked sex difference exists in visual attention to the mouth (figure 1a) between 2 and 20 months of life, and is most pronounced in early infancy through the first year, with females demonstrating an increase in mouth-looking that is greater in magnitude and steeper in slope than the parallel phenomenon in males. Sex-differences in visual attention to the mouth disappear, however, by 36 months. A concurrent difference in vocabulary production between females (M=0.25, SD=1.05) and males (M= -0.22, SD=0.89) at 18 months (t=4.02, p<0.001) converges by 36 months, at which point females (M=0.12, SD=0.86) no longer differ from males (M= -0.09, SD=1.09; t=1.14, p=0.256; figure 1b).  

Conclusions: Results indicate strong sex-specific differences in preferential visual attention beginning in very early infancy. When seen as an index of engagement with the social-communicative task of early language learning, female precociousness (increasing visual attention to the mouth more quickly than males in the first year) suggests an opportunity by which an existing sex difference of typical development might serve as a protective factor against developmental processes disrupted in ASD. Coupled together, the previously reported alignment of sex difference in mouth-fixation with sex difference in language acquisition, and the present study’s report of the disappearance of sex difference in mouth-fixation concurrent with the disappearance of sex difference in language indicate a role for female precociousness in communicative development (particularly in its earliest stages) as protective against insults to the typical trajectory of normative social development.