Table 2.

Descriptive characteristics and covariates for n = 1 050 271 (74%) index, second-born offspring in the sample of n = 1 418 820

Variable	n (%)
Second-born offspring	1 050 271
Interpregnancy interval (months)
0–5	27 888 (2.7)
6–11	128 096 (12.2)
12–23	369 173 (35.2)
24–35a	233 983 (22.3)
36–71	216 631 (20.6)
72+	74 500 (7.1)
Maternal age (years)
<20	6618 (0.6)
20–29a	596 101 (56.8)
30–39	433 634 (41.3)
≥40	13 918 (1.3)
Paternal age (years)
<20	1299 (0.1)
20–29a	390 057 (37.1)
30–39	576 730 (54.9)
≥40	82 185 (7.8)
Highest maternal education
≤9 years primary and lower secondarya	17 979 (1.7)
9 years primary and lower secondary	88 677 (8.4)
3 years upper secondary	514 757 (49.0)
Post-secondary/and or post-graduate	428 858 (40.8)
Highest paternal education
≤9 years primary and lower secondarya	43 960 (4.2)
9 years primary and lower secondary	131 470 (12.5)
3 years upper secondary	528 773 (50.4)
Post-secondary/and or post-graduate	346 068 (33.0)
Mother of Swedish nationality	933 506 (88.9)
Father of Swedish nationality	926 035 (88.2)
First- and second-born to different fathers	93 379 (8.9)
Maternal psychopathology
Criminality	109 882 (10.5)
Attempted suicide	23 099 (2.2)
Substance-use problem	17 300 (1.7)
Severe mental illness	8378 (0.8)
Paternal psychopathology
Criminality	393 941 (37.5)
Attempted suicide	16 520 (1.6)
Substance-use problem	33 079 (3.2)
Severe mental illness	6140 (0.6)

^adenotes reference category for model estimation.