Table I.

Means (SDs) for Sleep Problems at the Ages of 2 and 3 Years by Sex and Zygosity

	Males		Females		Effect size
Age	MZ twins	DZ twins	MZ twins	DZ twins	Age	Sex	Zygosity
Age 2	1.64 (1.99)	2.09 (2.71)	1.84 (2.41)	2.08 (2.37)	−.15	.01	−.19
n	147	183	138	152
Age 3	2.14 (2.43)	2.53 (2.87)	1.82 (2.01)	2.62 (2.69)
n	138	174	133	145

Note. Effect size was estimated by Cohen’s d. SP = sleep problems; MZ = monozygotic; DZ = dizygotic.