## 2 Methods  
### Participants  
The sample is composed of 29 infants (M/F:21/8) at increased likelihood of being diagnosed with ASD (IL) [39; 40] by virtue of having older siblings with ASD, which were compared to 27 infants (M/F:12/15) at reduced likelihood (RL) to be diagnosed with ASD. The sample was 96% Caucasian and 4% Hispanic. The maternal education was as follows: 12% secondary school, 42% high school and 46% university degree. This study had a longitudinal design, with disengagement of attention and JA assessed at 12 months (T0). T0 examiners, who administered the eye-tracking tasks and joint attention tasks, were different from T1 examiners, who administered the diagnostic assessment. At 24 months (T1), a diagnostic assessment was conducted by two different clinicians with extensive experience in the evaluation of toddlers with NDDs (a psychologist and a neuro-psychiatrist) within a tertiary care university hospital. The diagnostic assessment included the ADOS-2 Toddler Module administered by research-reliable raters [41], the Griffiths Mental Developmental Scales--ER (GMDS-ER, [42]) to obtain the developmental level, parental questionnaires and interviews (such as Vineland Adaptive Behavior Scale -II [43] and clinical observation. Neurological or sensory deficits were excluded by the neuro-psychiatrist through a clinical visit or through specific instrumental analysis, if needed. Anamnestic information (infant's history and family history) was collected by the neuro-psychiatrist as well. Clinical diagnosis was finally made according to the DSM-5 criteria. Blindness of group membership was possible for those who administered the eye-tracking measures and assessment measures (joint attention task and ADOS-2) but not for the neuropsychiatric, who was responsible for the collection of the anamnestic data. A total of 13 infants belonging to the RL group and 3 to the IL group did not complete the T1 assessment due to the March 2020 lockdown or dropped out from the study; for this reason, they were included only in the T0 analysis. At T1, IL and RL infants were classified into three groups according to their diagnostic status, including ASD (n = 10), NO-ASD (n = 25) or other neurodevelopmental disorders (NDDs) (n = 5, 3 with a language disorder and 2 with a neurodevelopmental disorder not otherwise specified). These latter, five infants with an NDD diagnosis were then excluded from the T1 analyses (see Figure 1 for a participant flowchart). IL infants were recruited from two distinct ongoing research projects at the IRCCS Fondazione Stella Maris (project 1: \"Early Bird Diagnostic Protocol for ASD\", code: NET -2013-02355263, and project 2: NIDA Network--Network Italiano per i Disturbi dello Spettro Autistico). Both projects focused on the developmental surveillance of infants at IL to develop ASD. Those projects involved experimental protocols at different developmental time points. Despite these differences, there were some overlaps in the procedures and in the measures that facilitated data sharing between the two projects. RL infants were recruited through local pediatricians, who enrolled infants without a family history of ASD and/or developmental concerns. The subjects were reimbursed for travel expenses incurred in reaching the center where the evaluation was carried out. The sample size was verified according to the existing literature [44; 45] and a power analysis using G*Power 3 [46], with a power level of 0.80, an effect size (f2) of 0.20, \(\alpha=0.025\) and an estimated sample size of 42 [47]. The initial sample consisted of 57 children; however, as reported in the manuscript, 13 children in the RL group and 3 in the IL group did not complete the T1 assessment due to the March 2020 lockdown or dropped out of the study. To assess the potential impact of this reduction, a post hoc power analysis was conducted, which indicated a reduction in power.