Table 1 Descriptive characteristics of the sample.

Variables	Overall	Boys	Girls
N at T₀	6469	3369	3100
Age, years at T₀	9.9 (0.6)	9.9 (0.6)	9.9 (0.6)
Socioeconomic status	0.01 (0.99)	0.03 (0.94)	-0.02 (1.00)
Polygenic score	0.02 (1.00)	0.04 (0.98)	-0.003 (1.02)
Digital media usage
Using social media
T₀	0.49 (1.06)^a	0.39 (0.95)^a	0.56 (1.09)^a
T₁	0.84 (1.49)^b	0.66 (1.31)^b	0.99 (1.59)^b
T₂	1.22 (1.33)^c	0.89 (1.16)^c	1.47 (1.38)^c
T₃	1.87 (1.45)^d	1.47 (1.35)^d	2.22 (1.45)^d
T₄	2.50 (1.45)	2.19 (1.49)	2.76 (1.36)
Watching television/videos
T₀	2.13 (1.69)^a	2.20 (1.72)^a	2.05 (1.65)^a
T₁	2.36 (1.77)^b	2.44 (1.80)^b	2.26 (1.73)^b
T₂	2.21 (1.24)	2.24 (1.24)	2.13 (1.23)
Playing video games
T₀	0.95 (1.06)^a	1.24 (1.15)^a	0.64 (0.84)^a
T₁	1.15 (1.18)^b	1.50 (1.23)^b	0.78 (0.99)^b
T₂	1.48 (1.39)^c	1.94 (1.37)^c	0.93 (1.19)^c
T₃	1.81 (1.47)^d	2.33 (1.36)^d	1.21 (1.35)^d
T₄	2.00 (1.57)	2.53 (1.40)	1.37 (1.51)
Average estimated time spent over four years on
Using social media	1.35 (0.95)	1.12 (0.87)	1.60 (0.99)
Watching television/videos	2.24 (1.27)	2.30 (1.28)	2.16 (1.25)
Playing video games	1.47 (1.02)	1.91 (0.96)	0.98 (0.86)
Playing mature video games at T₀	0.53 (0.84)	0.76 (0.94)	0.27 (0.60)
Watching mature movies at T₀	0.34 (0.60)	0.38 (0.62)	0.30 (0.58)
Total screen time—reported by children at
T₀	3.60 (2.93)	3.86 (2.99)	3.27 (2.76)
T₁	4.32 (3.38)	4.58 (3.33)	4.03 (3.42)
Total screen time—reported by parents at
T₀	2.90 (2.32)	3.04 (2.37)	2.73 (2.23)
T₁	3.17 (2.62)	3.28 (2.58)	3.05 (2.65)

Data are presented as the mean (standard deviation).
T₀, baseline visit; T₁, first-year visit; T₂, second-year visit, T₃, third-year visit; and T₄, fourth-year visit. A two-sample t-test was carried out to determine the differences between the visits.
^aSignificant differences between T₀ and T₁.
^bSignificant differences between T₁ and T₂.
^cSignificant differences between parent and child reported at T₀.
^dSignificant differences between parent and child reported at T₁.
^eSignificant differences between parent and child reported at T₂. The average estimated time spent over 2 years is calculated by the average across all three annual visits. The total screen time reported by children is calculated by summing the estimated time spent on individual digital media types. For example, total screen time at T₀ is calculated by summing the estimated time spent using social media, watching television and videos, and playing video games at T₀.

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