Table 3.

Standardised mean differences, d, in ninth grade test scores and teacher ratings, crude and adjusted for parental age and education, in mathematics, physics/chemistry, Danish and English by sex and co-twin sex for Danish twin and singleton females born 1986–1990. Positive values of d represent higher scores for males compared with females and/or opposite-sex twins compared with same-sex twins and/or opposite-/same-sex twins compared with singletons.

Characteristic	Male/females (ref. female)	Adjusted	Opposite-sex/ same-sex females (ref. same-sex)	Adjusted	Opposite-sex/ singleton females (ref. singletons)	Adjusted	Same-sex/ singleton females (ref. singletons)	Adjusted
Mathematics — oral
Test scores available	16,727	15,589	2540	2377	6726	6243	7688	7158
Effect size difference^c	.07	.06	−.09	−.13	−.07	−.13	.01	−.00
95% CI^d	(.04; .10)^b	(.03; .09)^b	(−.18; .00)	(−.21; −.04)^a	(−.15; .00)^a	(−.20; −.06)^b	(−.05; .07)	(−.06; .06)
Teacher ratings available	17,014	15,814	2565	2391	6840	6325	7817	7248
Effect size difference^c	−.02	−.04	−.08	−.12	−.06	−.11	.02	.01
95% CI^d	(−.05; .01)	(−.07; −.01)^a	(−.17; .01)	(−.20; −.03)^a	(−.13; .01)	(−.18; −.03)^a	(−.05; .08)	(−.05; .07)
Mathematics — written
Test scores available	16,777	15,627	2549	2383	6747	6254	7710	7171
Effect size difference^c	.17	.15	−.10	−.14	−.07	−.14	.02	−.00
95% CI^d	(.14; .20)^b	(.12; .18)^b	(−.19; −.01)^a	(−.23; −.05)^a	(−.15; .00)^a	(−.21; −.07)^b	(−.04; .09)	(−.06; .06)
Teacher ratings available	17,031	15,830	2570	2395	6849	6333	7827	7258
Effect size difference^c	−.01	−.03	−.08	−.12	−.07	−.14	.01	−.01
95% CI^d	(−.05; .02)	(−.06; .00)^a	(−.17; .01)	(−.21; −.03)^a	(−.15; .00)	(−.21; −.06)^b	(−.06; .07)	(−.07; .05)
Mathematics — neatness
Test scores available	13,086	12,168	1986	1857	5239	4835	6045	5600
Effect size difference^c	−.63	−.64	−.01	−.02	−.05	−.08	−.04	−.05
95% CI^d	(−.67; −.60)^b	(−.67; −.60)^b	(−.11; .09)	(−.12; .08)	(−.13; .04)	(−.17; .01)	(−.10; .03)	(−.12; .02)
Teacher ratings available	13,270	12,309	2002	1867	5307	4885	6129	5662
Effect size difference^c	−.71	−.72	−.08	−.12	−.05	−.12	.03	.00
95% CI^d	(−.74; −68)^b	(−.75; −.68)^b	(−.19;.02)	(−.22; −.01)^a	(−.14; .03)	(−.21; −.03)^a	(−.04; .10)	(−.07; .07)
Physics/chemistry — Oral
Test scores available	13,381	12,483	2037	1915	5314	4922	6125	5695
Effect size difference^c	.00	−.01	−.07	−.11	−.05	−.12	.02	−.00
95% CI^d	(−.04; .03)	(−.05; .02)	(−.18; .03)	(−.22; −.01)^a	(−.14; .03)	(−.20; −.04)^a	(−.05; .09)	(−.07; .07)
Teacher ratings available	16,267	15,149	2486	2320	6557	6069	7501	6965
Effect size difference^c	−.01	−.04	−.06	−.12	.00	−.06	.06	.05
95% CI^d	(−.04; .02)	(−.07; −.00)^a	(−.16; .03)	(−.21; −.03)^a	(−.07; .08)	(−.13; .01)	(−.00; .13)	(−.01; .11)
Danish — Oral
Test scores available	16,889	15,722	2581	2412	6825	6319	7810	7253
Effect size difference^c	−.33	−.35	.02	−.03	−.01	−.08	−.03	−.04
95% CI^d	(−.36; −30)^b	(−.38; −32)^b	(−.07; .11)	(−.12; .06)	(−.08; .06)	(−.15; −.01)^a	(−.09; .03)	(−.10; .02)
Teacher ratings available	17,034	15,833	2581	2407	6861	6345	7844	7276
Effect size difference^c	−.44	−.46	.03	−.01	.03	−.04	−.00	−.02
95% CI^d	(−.47; −41)^b	(−.49; −43)^b	(−.06; .13)	(−.10; .08)	(−.04; .10)	(−.11; .03)	(−.07; .06)	(−.08; .04)
Danish — Written
Test scores available	16,887	15,720	2583	2413	6829	6323	7806	7250
Effect size difference^c	−.48	−.49	.04	−.02	−.02	−.09	−.06	−.07
95% CI^d	(−.51; −45)^b	(−.52; −46)^b	(−.05; .13)	(−.10; .07)	(−.10; .05)	(−.17; −.02)^a	(−.12; .00)	(−13; −.01)^a
Teacher ratings available	17,033	15,831	2581	2407	6861	6345	7844	7276
Effect size difference^c	−.58	−.60	−.02	−.06	−.06	−.13	−.05	−.07
95% CI^d	(−.61; −55)^b	(−.63; −57)^b	(−.11; .08)	(−.15; .03)	(−.14; .01)	(−.20; −.06)^b	(−.11; .02)	(−.13; −.01)^a
Danish — spelling
Test scores available	16,930	15,758	2591	2420	6846	6335	7827	7267
Effect size difference^c	−.31	−.33	−.02	−.05	−.03	−.08	−.02	−.04
95% CI^d	(−.35; −28)^b	(−.36; −30)^b	(−.11; .08)	(−.14; .04)	(−.11; .04)	(−.16; −.01)^a	(−.08; .04)	(−.10; .02)
Teacher ratings available	17,023	15,827	2577	2405	6857	6342	7838	7273
Effect size difference^c	−.42	−.43	−.02	−.06	−.03	−.08	−.01	−.03
95% CI^d	(−.45; −39)^b	(−.46; −40)^b	(−.11; .08)	(−.15; .03)	(−.11; .04)	(−.15; −01)^a	(−.08; .05)	(−.09; .03)
Danish — Neatness
Test scores available	16,883	15,716	2582	2412	6828	6322	7804	7248
Effect size difference^c	−.44	−.45	−.01	−.03	−.04	−.07	−.03	−.04
95% CI^d	(−.47; −41)^b	(−.48; −41)^b	(−.10; .08)	(−.12; .06)	(−.12; .03)	(−.15; .00)	(−.09; .03)	(−.10; .02)
Teacher ratings available	17,019	15,819	2580	2406	6854	6338	7836	7268
Effect size difference^c	−.66	−.67	−.04	−.07	−.07	−.11	−.03	−.04
95% CI^d	(−.69; −63)^b	(−.70; −64)^b	(−.13; .05)	(−.16; .02)	(−.14; .01)	(−.18; −03)^a	(−.09; .04)	(−.10; .03)
English — oral
Test scores available	16,286	15,197	2485	2328	6574	6100	7529	7008
Effect size difference^c	−.18	−.20	.03	−.03	.01	−.07	−.02	−.03
95% CI^d	(−.21; −15)^b	(−.23; −17)^b	(−.06; .13)	(−.12; .06)	(−.06; .09)	(−.15; .00)	(−.08; .04)	(−.09; .03)
Teacher ratings available	16,851	15,682	2550	2380	6799	6291	7765	7209
Effect size difference^c	−.25	−.27	.03	−.03	.04	−.05	.01	−.01
95% CI^d	(−.28; −22)^b	(−.30; −.24)^b	(−.07; .12)	(−.11; .06)	(−.04; .11)	(−.12; .03)	(−.05; .08)	(−.07; .05)

p < 0.05.

p < 0.001.

In units of SD.

Confidence intervals.