The association between screen time and genetic risks for neurodevelopmental disorders in children

Whether longer screen time in infancy increases risk of neurodevelopmental disorders (NDDs) such as autism spectrum disorder (ASD) and ADHD has long been debated, but no causal relationship between the two remains has been established. Using ongoing longitudinal cohort data, we found that in children 24 to 40 months of age, the genetic risk of ASD was associated with longer screen time and that of ADHD with an increase in screen time over time. These data suggest that prolonged screen time may not be a cause of the genetic risk for NDD, but an early sign of NDDs.


Introduction
Recently, there has been much discussion regarding whether long screen time in infancy increases the risk of neurodevelopmental disorders (Kushima et al., 2022). Although the results are inconsistent, methods for reducing screen time continue to be widely discussed. One study suggested that reducing screen time protects against the development of neurodevelopmental disorders 1 while another study, by Sugiyama and Tsuchiya, reported that duration of screen time was not associated with social interaction difficulties (Sugiyama et al., 2023). Considering the variations in the studies, it is difficult to conclude that a causal relationship exists between screen time and neurodevelopmental disorders.
To further understand the relationship, this study examined the association between genetic risks for neurodevelopmental disorders and screen time. We used data from a long-term observational cohort study rather than using single-point measurements to identify factors influencing screen time in children. Using ongoing longitudinal cohort data is important in order to identify the trajectories of screen time in children, and explore whether genetic risks for neurodevelopmental disorders are correlated with the trajectories.

Methods
Participants of the ongoing Hamamatsu Birth Cohort for Mothers and Children study conducted in Hamamatsu were enrolled in this study. All subjects were of Japanese descent. Data on screen time was gathered at 24, 32, and 40 months of age by the parents of the participating children, who were then interviewed using the Environmental Questionnaire developed by The International Study of Asthma and Allergies in Childhood. Details are in our previous paper (Sugiyama et al., 2023).
The genetic risk for ASD (ASD polygenic risk score [ASD-PRS]) and ADHD (ADHD polygenic risk score [ADHD-PRS]) was calculated using summary data from recent genome-wide association studies conducted by the Psychiatric Genomics Consortium (Takahashi et al., 2020). PRSice-2 was used for calculating polygenic risk scores (PRS) (Choi and O'Reilly, 2019). Detailed methods are described in our previous papers (Takahashi et al., 2022). Daily screen time was classified according to the categories used by Kushima 1 (Level 1, <1 h; Level 2, 1-2 h, Level 3, 2-4 h; Level 4, >4 h). Trajectory analysis was conducted using the traj package of Stata 16 software (StataCorp. 2019. Stata Statistical Software: Release 16. College Station, TX: StataCorp LLC.). Model 1 includes ASD-PRS and ADHD-PRS as risk factors and sex and number of siblings were included as covariates in Model 2 since these factors are proposed as ones affecting screen time (Hedderson et al., 2023;Langoy et al., 2019). Multinomial logistic regression was used to identify the risk and protective factors for group trajectory. Statistical significance was set at P < 0.05.
This study was approved by the ethics committee of Hamamatsu University School of Medicine (research ID:17-037 and 19-145) and reported according to the Strengthening the Reporting of Observational Studies in Epidemiology guidelines.

Results
A total of 437 participants (224 males and, 213 females) were analysed. The trajectory analysis showed that the screen time of 27.9% of the children was < 1 h (Group 1; level 1 at all three time points), that of 19.0% of children increased over time (Group 2; level 2 at 24 months, level 3 or 4 after 32 months of age), that of 20.3% of children decreased over time (Group 3; level 3 at 24 months of age, level 2 or 1 after 32 months of age) and that of 32.  Table 3).

Discussion
In this study, children <40 months of age showed different trajectories of daily screen time over time. Additionally, ASD-PRS was associated with medium and long screen time during the observation period.
This raises the possibility that long screen time is not a cause of ASD, but rather an early symptom that ASD individuals are more attracted to objects than to people. Conversely, the ADHD-PRS was found to be associated with changes in screen time during the observation period. ADHD-PRS was associated with increase in screen time, which was potentially thought to be associated with ADHD subjects' susceptibility to gaming addiction.
Parents of children with neurodevelopmental disorders are often criticized for allowing their children too much screen time in order to reduce parenting difficulties, which may deprive the children of opportunities to acquire language and social skills (Lissak, 2018;Trinh et al., 2020). To our knowledge, this is the first study to show that genetic risks for neurodevelopmental disorders may affect the length of screen time in children. Thus, caregivers of children with neurodevelopmental disorders should be helped and offered alternative behavioural management strategies.
One of the most encouraging findings in this study is the possibility of preventing longer and increasing screen time in children with siblings.
Although the impact of screen time itself on the development of neurodevelopmental disorders is still inconclusive, it seems likely that having siblings may increase outdoor play and the development of social skills (Sugiyama et al., 2023).
The present study has some limitations. First, screen time was evaluated by parental reports which may have resulted in underestimation. Second, we did not classified screen time in this analysis, for example, TV watching, video game watching, and YouTube watching etc. Third, because the participants in this study ranged from 24 to 40 months of age, it was not possible to examine the effects of ASD and ADHD symptom severity. Finally, since we do not have diagnosis data for ASD and ADHD from a health specialist or clinician, caution is needed in interpretation of the present findings for potential intervention. Further research and replication studies using larger sample are needed.

Funding
This work was supported by grants from the Ministry of Education, Culture, Sports, Science & Technology in Japan (Grant no. 19H03582, 21KK0145, and 22H00492 to KJT).

Role of the funder
The funders have no roles in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Abbreviations: ST, screen time; ASD, autism spectrum disorder; ADHD, attention deficit hyperactivity disorder; PRS, polygenic risk score; OR, odds ratio. *indicates P-value <0.05, **indicates P-value <0.01, ***indicates P-value <0.001.
The authors, Nagahide Takahashi and Kenji J. Tsuchiya, had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.