Table 1. Descriptive characteristics and covariates for the index, second-born offspring.

Variable	n (%)
Second-born offspring
Interpregnancy Interval (months)
0-5	27888 (2.7)
6-11	128096 (12.2)
12-17	202011 (19.2)
18-23*	166374 (15.8)
24-59	414715 (39.5)
60+	111187 (10.6)
Maternal Age (yrs)
< 20	6618 (0.6)
20-29*	596101 (56.8)
30-39	433634 (41.29)
≥ 40	13918 (1.3)
Paternal Age (yrs)
< 20	1299 (0.1)
20-29*	390057 (37.1)
30-39	576730 (54.9)
≥ 40	82185 (7.8)
Highest Maternal Education
≤ 9 years primary and lower secondary*	17979 (1.7)
9 years primary and lower secondary	88677 (8.4)
3 years upper secondary	514757 (49.0)
Post-secondary/and or postgraduate	428858 (40.8)
Highest Paternal Education
≤ 9 years primary and lower secondary*	43960 (4.2)
9 years primary and lower secondary	131470(12.5)
3 years upper secondary	528773 (50.4)
Post-secondary/and or postgraduate	346068 (33.0)
Mother of Swedish Nationality	933506(88.9)
Father of Swedish Nationality	926035 (88.2)
First- and Second-born to Different Fathers	93379 (8.9)
Maternal Psychopathology
Criminality	109882 (10.5)
Substance use problem	23099 (2.2)
Attempted suicide	17300 (1.7)
Severe mental illness	8378(0.8)
Paternal Psychopathology
Criminality	393941(37.5)
Substance use problem	16520(1.6)
Attempted suicide	33079(3.2)
Severe mental illness	6140(0.6)
First-born preterm	56394(5.4)
First-born low birth weight	39238(3.7)
First-born small for gestational age	39297(3.7)

Note:

^*reference category