Table 1.

Descriptive statistics.

Baseline characteristics at ages 12-13 years (N=3228)				Participants, n (%)			Mean (SD)
	Active social media use on weekdays
		None	828 (25.7)		N/Aa
		Less than an hour	1012 (31.4)		N/A
		1-3 hours	1029 (31.9)		N/A
		4-6 hours	264 (8.2)		N/A
		7 or more hours	95 (2.9)		N/A
	Poor mental health—SDQb,c		N/A			10.45 (5.64)
	Self-esteemd		N/A			3.19 (0.42)
	Number of close friends		N/A			7.53 (7.69)
	Happiness with friendse		N/A			6.37 (0.95)
	Gender
		Female	1659 (51.4)		N/A
		Male	1569 (48.6)		N/A
	Ethnicity
		White	2522 (78.1)		N/A
		Black African or Caribbean	146 (4.5)		N/A
		Asian	395 (12.2)		N/A
		Mixed	154 (4.8)		N/A
		Other	11 (0.3)		N/A
	Baseline year
		2009-2011	1756 (54.40)		N/A
		2012-2014	809 (25.06)		N/A
		2015-2017	663 (20.54)		N/A
	Mother’s highest qualification
		No qualification	470 (17.8)		N/A
		GCSEf or equivalentg	882 (33.5)		N/A
		A/ASh-level or equivalent	250 (9.5)		N/A
		Other higher qualification	373 (14.2)		N/A
		Degree	661 (25.1)		N/A
	Mother’s marital status
		Nonpartnered	603 (19.8)		N/A
		Partnered	2441 (80.2)		N/A
	Number of employed people in household
		None employed	332 (10.3)		N/A
		1 employed	1059 (32.8)		N/A
		2 employed	1620 (50.2)		N/A
		>2 employed	217 (6.7)		N/A
	OECDi equalized household incomej (£k)		N/A			1474.54 (971.88)
Mediators at ages 13-14 years (n=2642)
	Self-esteem		N/A			3.11 (0.44)
	Number of close friends		N/A			7.20 (10.26)
	How do you feel about friends?		N/A			6.22 (1.00)
Outcome at ages 14-15 years (n=3228)
	Poor mental health—SDQ		N/A			10.91 (5.81)

^aN/A: not applicable.

^bScores range from 0-40, with higher scores equating to more mental health problems.

^cSDQ: Strengths and Difficulties Questionnaire.

^dScores range 1-4, higher scores equate to higher self-esteem.

^eScores range 1-7, higher scores equate to more happiness with friends.

^fGCSE: General Certificate of Secondary Education.

^gComparison to US qualifications.

^hA/AS level: advanced/advanced subsidiary.

ⁱOECD: Organization for Economic Co-operation and Development

^jFor international income comparisons see the OECD website.

^kUS $1=0.85 GBP.