Table S1.

Comparison Between Participating and Nonparticipating Families

	Participating		Nonparticipating		t Test	d	p
	n	Mean (SD)	n	Mean (SD)
Socio-economic status at first contact	3695	0.25 (0.98)	3722	–0.09 (0.97)	–15.09	.35	<.001
Natural mother’s age at birth of twins	3943	31.47 (4.57)	4290	30.60 (4.96)	–8.27	.18	<.001
Natural father’s age at birth of twins	3651	33.84 (5.67)	3757	33.30 (5.95)	–3.99	.09	<.001
	n	%	n	%	χ2	φc	p
Ethnicity of twins					15.18	.04	<.001
White	3726	93.45%	3996	91.15%
Other	261	6.55%	388	8.85%
Proportion with cohabiting parents at first contact	3738	93.54%	3895	88.58%	64.29	.09	<.001
Has higher qualification
Mothers	1172	29.72%	863	19.98%	104.72	.11	<.001
Fathers	1473	40.06%	1073	28.33%	113.78	.12	<.001
Mother’s employment status at first contact					54.48	.08	<.001
Unemployed	397	10.00%	662	15.20%
Employed	1888	47.56%	1866	42.84%
Staying home to look after children	1685	42.44%	1828	41.96%
Father’s employment status at first contact					23.13	.06	<.001
Unemployed	154	4.13%	251	6.53%
Employed	3499	93.86%	3503	91.08%
Staying home to look after children	75	2.01%	92	2.39%

Note: Higher qualification defined as those who have post-school qualifications (usually after the age of 18) including higher diploma/certificate, undergraduate degree, or postgraduate degrees; d = Cohen’s d where small effect d = 0.2, medium d = 0.5, and large d = 0.8; φ_c = Phi, an estimation of effect size appropriate for χ² tests with small effect φ_c = 0.1, medium φ_c = 0.3, and large φ_c = 0.5.