The effect of number of siblings on adult mortality: Evidence from Swedish registers for cohorts born between 1938 and 1972

Figures & data

Table 1 Descriptive statistics for analysis population of Swedish men and women born 1938–72

Variable	Category	Men			Women
		Person-months (%)	Deaths	Mortality rate (10^−4)	Person-months (%)	Deaths	Mortality rate (10^−4)
Sibling group size	1	13.3	14,485	4.01	13.4	9,398	2.67
	2	35.7	29,715	3.05	35.8	19,355	2.06
	3	26.3	21,423	3.00	25.9	13,973	2.05
	4	13.1	11,862	3.32	13.1	7,426	2.16
	5	5.9	5,971	3.70	6.0	3,798	2.41
	6	2.9	2,929	3.73	2.9	1,923	2.51
	7	1.5	1,631	4.09	1.5	997	2.51
	8	0.8	805	3.89	0.8	588	2.89
	9	0.4	417	3.89	0.4	305	2.76
	10	0.2	231	3.97	0.2	154	2.84
Cohort	1938–44	31.8	43,327	5.00	32.2	28,095	3.32
	1945–49	24.4	23,210	3.49	24.4	15,158	2.36
	1950–54	17.2	12,321	2.63	17.1	7,770	1.73
	1955–59	12.6	6,314	1.85	12.5	4,138	1.26
	1960–64	8.4	3,028	1.32	8.4	1,893	0.86
	1965–69	4.8	1,139	0.87	4.7	778	0.63
	1970–72	0.7	130	0.67	0.7	85	0.46
Maternal age	15–19	3.8	3,568	3.41	3.9	2,306	2.27
	20–24	23.3	21,106	3.32	23.4	13,317	2.16
	25–29	31.1	26,710	3.15	31.0	17,291	2.13
	30–34	23.7	21,089	3.27	23.6	13,815	2.23
	35–39	13.3	12,381	3.41	13.3	8,051	2.30
	40+	4.7	4,615	3.60	4.8	3,137	2.47
Birth order	1	46.8	44,028	3.46	46.8	28,327	2.30
	2	32.1	27,419	3.13	32.0	17,720	2.11
	3	13.1	11,157	3.12	13.2	7,399	2.14
	4	4.9	4,148	3.13	4.9	2,672	2.08
	5	1.8	1,630	3.25	1.9	1,062	2.17
	6	0.7	640	3.15	0.8	441	2.21
	7	0.3	282	3.26	0.3	189	2.23
	8	0.1	112	3.00	0.1	67	1.96
	9	0.0	39	3.00	0.1	34	2.34
	10	0.0	14	3.52	0.0	6	1.48
Parental social class	I	1.9	1,520	2.97	1.8	1,020	2.12
	II	23.3	17,022	2.68	22.9	11,169	1.86
	III	1.6	1,630	3.82	1.6	1,096	2.62
	IV	20.3	17,813	3.22	20.2	11,289	2.13
	V–VII	45.6	44,532	3.59	46.2	28,796	2.37
	Unknown/other	7.3	6,952	3.49	7.3	4,547	2.37
Total person-months		272,290,421			262,787,225
Total deaths			89,469			57,917

Note: Parental social class categorized according to the Erikson/Goldthorpe/Portocarero scheme.

Source: Swedish register data for 1932–2012.

Table 2 Distribution of men and women in Sweden across sibling group sizes by cohort (per cent) and infant mortality (number of deaths per 100,000 births)

Sibling group sizes
Cohort (men)	1	2	3	4	5	6	7	8	9	10	Total	Infant mortality
1938–44	17.6	32.7	23.6	12.9	6.5	3.3	1.8	0.9	0.5	0.3	100.0	2,196
1945–49	14.3	34.9	25.2	13.2	6.2	3.1	1.6	0.8	0.4	0.2	100.0	1,922
1950–54	11.2	35.5	27.0	13.9	6.3	3.1	1.5	0.8	0.4	0.2	100.0	1,288
1955–59	9.3	36.8	29.2	14.1	5.7	2.6	1.2	0.6	0.3	0.2	100.0	1,052
1960–64	7.7	41.1	30.9	12.6	4.4	1.8	0.8	0.4	0.2	0.1	100.0	986
1965–69	7.7	47.5	30.5	9.7	2.9	1.0	0.4	0.2	0.1	0.1	100.0	875
1970–72	8.0	50.9	29.4	8.3	2.2	0.7	0.3	0.1	0.1	0.0	100.0	728
Cohort (women)
1938–44	17.6	32.8	23.3	12.9	6.6	3.3	1.8	0.9	0.5	0.2	100.0	1,582
1945–49	14.3	34.9	24.9	13.2	6.3	3.2	1.6	0.9	0.5	0.2	100.0	1,397
1950–54	11.3	35.5	26.6	14.0	6.5	3.1	1.6	0.8	0.4	0.2	100.0	919
1955–59	9.5	37.1	28.9	14.0	5.7	2.6	1.2	0.6	0.3	0.2	100.0	762
1960–64	7.9	41.4	30.6	12.4	4.4	1.8	0.8	0.4	0.2	0.1	100.0	710
1965–69	7.9	47.6	30.2	9.7	2.8	1.1	0.4	0.2	0.1	0.1	100.0	623
1970–72	7.8	51.3	29.2	8.1	2.3	0.8	0.3	0.1	0.0	0.0	100.0	504

Source: As for Table 1.

Table 3 Correlation of twin births with sibling group size according to birth order of twins

	Men, birth order of twin birth
	2	3	4	5
Correlation of twin births with sibling group size	0.733	0.818	0.811	0.804
Number of twins	4,888	4,569	2,500	1,279
	Women, birth order of twin birth
	2	3	4	5
Correlation of twin births with sibling group size	0.741	0.803	0.800	0.800
Number of twins	4,630	4,237	2,330	1,217

Source: As for Table 1.

Table 4 Results: sibling group size and mortality among Swedish men born 1938–72, based on Gompertz shared frailty hazard model analysis

Variable	Category	Model 1			Model 2			Model 3
		RR	SE	95% CI	RR	SE	95% CI	RR	SE	95% CI
Sibling group size	1	1.13	0.01	1.11–1.16	1.17	0.01	1.1–1.20	1.14	0.01	1.12–1.17
	2	1.00			1.00			1.00
	3	0.98	0.01	0.96–1.00	0.96	0.01	0.94–0.97	0.97	0.01	0.95–0.98
	4	1.04	0.01	1.02–1.06	0.99	0.01	0.97–1.02	1.00	0.01	0.98–1.03
	5	1.11	0.02	1.07–1.14	1.04	0.02	1.01–1.07	1.04	0.02	1.01–1.08
	6	1.09	0.02	1.05–1.14	1.01	0.02	0.96–1.05	1.00	0.02	0.96–1.05
	7	1.18	0.03	1.12–1.25	1.07	0.03	1.01–1.13	1.06	0.03	1.00–1.13
	8	1.11	0.04	1.03–1.20	0.99	0.04	0.91–1.07	0.98	0.04	0.90–1.07
	9	1.09	0.06	0.97–1.21	0.94	0.06	0.84–1.06	0.94	0.05	0.84–1.06
	10	1.13	0.08	0.98–1.31	0.96	0.08	0.82–1.12	0.95	0.08	0.82–1.11
Cohort	1938–44	1.00			1.00			1.00
	1945–49	1.01	0.01	0.99–1.03	1.00	0.01	0.98–1.02	1.01	0.01	0.99–1.02
	1950–54	1.05	0.01	1.03–1.07	1.03	0.01	1.01–1.05	1.04	0.01	1.02–1.06
	1955–59	1.01	0.02	0.98–1.04	0.99	0.01	0.96–1.02	1.00	0.02	0.97–1.03
	1960–64	0.97	0.02	0.93–1.01	0.95	0.02	0.91–0.99	0.94	0.02	0.90–0.98
	1965–69	0.84	0.03	0.79–0.89	0.82	0.03	0.77–0.87	0.79	0.02	0.74–0.84
	1970–72	0.80	0.07	0.67–0.95	0.78	0.07	0.66–0.93	0.75	0.07	0.63–0.89
Maternal age	15–19	1.21	0.02	1.17–1.25	1.26	0.02	1.22–1.31	1.20	0.02	1.16–1.25
	20–24	1.09	0.01	1.07–1.12	1.12	0.01	1.10–1.14	1.09	0.01	1.07–1.11
	25–29	1.00			1.00			1.00
	30–34	0.99	0.01	0.97–1.01	0.97	0.01	0.95–0.99	0.98	0.01	0.96–1.00
	35–39	1.01	0.01	0.99–1.03	0.97	0.01	0.95–1.00	0.98	0.01	0.96–1.01
	40+	1.05	0.02	1.02–1.09	1.00	0.02	0.97–1.04	1.01	0.02	0.97–1.04
Birth order	1				1.00			1.00
	2				1.07	0.01	1.05–1.09	1.05	0.01	1.04–1.07
	3				1.13	0.02	1.10–1.16	1.11	0.02	1.08–1.14
	4				1.14	0.02	1.09–1.18	1.11	0.02	1.06–1.15
	5				1.20	0.04	1.13–1.27	1.16	0.04	1.09–1.23
	6				1.24	0.06	1.13–1.36	1.20	0.05	1.09–1.31
	7				1.35	0.09	1.19–1.54	1.30	0.09	1.14–1.48
	8				1.35	0.14	1.11–1.65	1.30	0.13	1.06–1.59
	9				1.50	0.25	1.08–2.10	1.44	0.24	1.04–2.01
	10				1.87	0.53	1.08–3.25	1.81	0.51	1.04–3.13
Parental social class	I							1.00
	II							1.03	0.03	0.97–1.09
	III							1.26	0.05	1.17–1.36
	IV							1.03	0.03	0.98–1.09
	V–VII							1.28	0.03	1.21–1.35
	Unknown/other							1.35	0.04	1.27–1.43
Number of men		1,354,253			1,354,253			1,354,253
Deaths		89,469			89,469			89,469

Notes: Parental social class categorized according to the Erikson/Goldthorpe/Portocarero scheme. RR indicates relative risk (the reference category is the category for which RR equals one), SE indicates standard errors, and 95% CI indicates 95 per cent confidence intervals.

Source: As for Table 1.

Figure 1 The relative risk of mortality by sibling group size among men born in Sweden from 1938 to 1972

Note: Bars indicate 95 per cent confidence intervals.Source: As for Table 1.

Table 5 Results: sibling group size and mortality among Swedish women born 1938–72, based on Gompertz shared frailty hazard model analysis

Variable	Category	Model 1			Model 2			Model 3
		RR	SE	95% CI	RR	SE	95% CI	RR	SE	95% CI
Sibling group size	1	1.12	0.01	1.09–1.15	1.15	0.02	1.12–1.19	1.13	0.02	1.10–1.16
	2	1.00			1.00			1.00
	3	1.00	0.01	0.98–1.02	0.97	0.01	0.94–0.99	0.98	0.01	0.95–1.00
	4	1.00	0.01	0.98–1.03	0.95	0.01	0.92–0.98	0.96	0.02	0.93–0.99
	5	1.07	0.02	1.03–1.11	0.99	0.02	0.96–1.04	1.00	0.02	0.96–1.04
	6	1.09	0.03	1.04–1.14	0.99	0.03	0.94–1.05	1.00	0.03	0.95–1.06
	7	1.06	0.04	0.99–1.13	0.95	0.04	0.88–1.02	0.95	0.04	0.88–1.02
	8	1.23	0.06	1.12–1.34	1.09	0.05	0.99–1.20	1.09	0.05	0.99–1.20
	9	1.16	0.07	1.02–1.31	1.00	0.07	0.88–1.15	1.01	0.07	0.88–1.15
	10	1.23	0.11	1.04–1.47	1.07	0.10	0.89–1.29	1.07	0.10	0.89–1.29
Cohort	1938–44	1.00			1.00			1.00
	1945–49	1.05	0.01	1.03–1.07	1.03	0.01	1.01–1.06	1.04	0.01	1.02–1.06
	1950–54	1.06	0.01	1.03–1.09	1.04	0.01	1.01–1.07	1.05	0.02	1.02–1.08
	1955–59	1.06	0.02	1.02–1.10	1.04	0.02	1.00–1.08	1.04	0.02	1.01–1.08
	1960–64	0.98	0.03	0.93–1.03	0.96	0.02	0.91–1.01	0.94	0.02	0.90–0.99
	1965–69	0.93	0.04	0.86–1.00	0.91	0.03	0.84–0.98	0.88	0.03	0.81–0.95
	1970–72	0.85	0.09	0.69–1.06	0.83	0.09	0.67–1.03	0.80	0.09	0.65–0.99
Maternal age	15–19	1.19	0.03	1.14–1.24	1.24	0.03	1.19–1.30	1.19	0.03	1.14–1.25
	20–24	1.05	0.01	1.03–1.08	1.08	0.01	1.05–1.10	1.05	0.01	1.03–1.08
	25–29	1.00			1.00			1.00
	30–34	1.00	0.01	0.98–1.03	0.98	0.01	0.96–1.01	0.99	0.01	0.97–1.01
	35–39	1.01	0.01	0.98–1.04	0.97	0.01	0.95–1.00	0.98	0.01	0.96–1.01
	40+	1.07	0.02	1.03–1.11	1.02	0.02	0.98–1.06	1.02	0.02	0.98–1.07
Birth order	1				1.00			1.00
	2				1.07	0.01	1.04–1.09	1.05	0.01	1.03–1.08
	3				1.15	0.02	1.11–1.19	1.13	0.02	1.09–1.17
	4				1.15	0.03	1.09–1.20	1.12	0.03	1.06–1.18
	5				1.21	0.05	1.13–1.30	1.18	0.04	1.09–1.27
	6				1.27	0.07	1.14–1.42	1.23	0.07	1.11–1.37
	7				1.34	0.11	1.14–1.57	1.30	0.10	1.11–1.52
	8				1.20	0.16	0.93–1.54	1.16	0.15	0.90–1.50
	9				1.55	0.28	1.08–2.21	1.50	0.27	1.05–2.15
	10				1.01	0.43	0.44–2.30	0.98	0.41	0.43–2.23
Parental social class	I							1.00
	II							0.99	0.03	0.93–1.06
	III							1.20	0.05	1.10–1.31
	IV							0.95	0.03	0.89–1.02
	V–VI							1.18	0.04	1.10–1.26
	Unknown/other							1.25	0.04	1.17–1.34
Number of women		1,284,645			1,284,645			1,284,645
Deaths		57,917			57,917			57,917

Notes: Parental social class categorized according to the Erikson/Goldthorpe/Portocarero scheme. RR indicates relative risk (the reference category is the category for which RR equals one), SE indicates standard errors, and 95% CI indicates 95 per cent confidence intervals.

Source: As for Table 1.

Figure 2 The relative risk of mortality by sibling group size among women born in Sweden from 1938 to 1972

Note: Bars indicate 95 per cent confidence intervals. Source: As for Table 1.

Figure 3 The relative risk of cause-specific mortality by sibling group size among men born in Sweden from 1938 to 1972

Notes: Estimates based on survival analyses controlling for cohort, maternal age, birth order, and parental SES. Bars indicate 95 per cent confidence intervals. Results for sibling group sizes 7–10 not displayed. Source: As for Table 1.

Figure 4 The relative risk of cause-specific mortality by sibling group size among women born in Sweden from 1938 to 1972

Notes: Estimates based on survival analyses controlling for cohort, maternal age, birth order, and parental SES. Bars indicate 95 per cent confidence intervals. Results for sibling group sizes 7–10 not displayed. Source: As for Table 1.

Figure 5 The relative risk of all-cause mortality by sibling group size according to the sex of siblings, for people born in Sweden from 1938 to 1972

Notes: Estimates based on survival analyses controlling for cohort, maternal age, birth order, and parental SES. Bars indicate 95 per cent confidence intervals. Results for sibling group sizes 7–10 not displayed. Source: As for Table 1.

Table 6 Results: birth of subsequent twin siblings and mortality among first-, second-, third-, and fourth-born Swedish men born 1938–72, based on Gompertz hazard model analysis

Variable	Category	First-born sample			First- and second-born sample			First-, second-, and third-born sample			First-, second-, third-, and fourth-born sample
		RR	SE	95% CI	RR	SE	95% CI	RR	SE	95% CI	RR	SE	95% CI
Subsequent twin birth	No	1.00			1.00			1.00			1.00
	Yes	1.04	0.06	0.92–1.16	0.92	0.06	0.82–1.04	0.96	0.07	0.83–1.11	0.82	0.08	0.67–0.99
Cohort	1938–44	1.00			1.00			1.00			1.00
	1945–49	0.97	0.01	0.94–1.00	1.00	0.02	0.97–1.03	1.02	0.02	0.98–1.06	1.06	0.03	1.01–1.12
	1950–54	0.98	0.02	0.94–1.02	1.02	0.02	0.98–1.07	1.06	0.03	1.01–1.12	1.07	0.04	1.00–1.15
	1955–59	0.94	0.02	0.89–0.99	0.97	0.03	0.92–1.02	1.01	0.04	0.94–1.08	1.00	0.05	0.90–1.11
	1960–64	0.85	0.03	0.80–0.92	0.85	0.03	0.79–0.92	0.89	0.05	0.79–0.99	1.00	0.09	0.85–1.19
	1965–69	0.68	0.04	0.61–0.76	0.73	0.05	0.65–0.83	0.69	0.08	0.55–0.85	0.94	0.16	0.68–1.30
	1970–72	0.63	0.10	0.47–0.86	0.58	0.12	0.39–0.86	1.07	0.29	0.63–1.80	0.51	0.36	0.13–2.06
Maternal age	15–19	1.22	0.03	1.17–1.28	1.19	0.03	1.14–1.25	1.16	0.04	1.08–1.23	1.19	0.05	1.09–1.30
	20–24	1.11	0.02	1.08–1.14	1.09	0.02	1.06–1.13	1.10	0.02	1.06–1.14	1.09	0.03	1.03–1.14
	25–29	1.00			1.00			1.00			1.00
	30–34	1.01	0.02	0.98–1.05	0.97	0.02	0.94–1.01	0.99	0.02	0.95–1.04	0.98	0.03	0.92–1.03
	35–39	0.98	0.03	0.92–1.04	1.02	0.03	0.97–1.09	1.00	0.03	0.93–1.06	1.07	0.04	0.99–1.17
	40+	1.02	0.09	0.85–1.22	1.15	0.09	0.98–1.34	1.05	0.09	0.88–1.26	1.11	0.11	0.91–1.34
Parental social class	I	1.00			1.00			1.00			1.00
	II	0.96	0.04	0.88–1.04	1.01	0.05	0.92–1.10	1.03	0.07	0.90–1.18	0.82	0.09	0.66–1.01
	III	1.10	0.07	0.98–1.24	1.22	0.08	1.07–1.39	1.30	0.12	1.08–1.56	1.09	0.16	0.82–1.44
	IV	0.96	0.04	0.89–1.05	1.01	0.05	0.92–1.10	1.02	0.07	0.90–1.17	0.87	0.09	0.71–1.08
	V–VII	1.19	0.05	1.10–1.29	1.29	0.06	1.18–1.41	1.37	0.09	1.20–1.56	1.13	0.12	0.92–1.40
	Unknown/other	1.21	0.06	1.10–1.33	1.29	0.06	1.17–1.42	1.41	0.10	1.22–1.63	1.22	0.14	0.98–1.52
Number of person months		457,944			382,757			192,848			91,149
Deaths		29,543			27,247			16,981			9,267

Notes: Parental social class categorized according to the Erikson/Goldthorpe/Portocarero scheme. RR indicates relative risk (the reference category is the category for which RR equals one), SE indicates standard errors, and 95% CI indicates 95 per cent confidence intervals.

Source: As for Table 1.

Table 7 Results: birth of subsequent twin siblings and mortality among first-, second-, third-, and fourth-born Swedish women born 1938–72, based on Gompertz hazard model analysis

Variable	Category	First-born sample			First- and second-born sample			First-, second-, and third-born sample			First-, second-, third-, and fourth-born sample
		RR	SE	95% CI	RR	SE	95% CI	RR	SE	95% CI	RR	SE	95% CI
Subsequent twin birth	No	1.00			1.00			1.00			1.00
	Yes	1.08	0.07	0.94–1.23	1.07	0.08	0.93–1.23	1.09	0.09	0.92–1.30	1.12	0.12	0.91–1.39
Cohort	1938–44	1.00			1.00			1.00			1.00
	1945–49	1.01	0.02	0.98–1.05	1.03	0.02	0.99–1.07	1.03	0.03	0.98–1.08	1.04	0.03	0.97–1.11
	1950–54	1.01	0.03	0.96–1.06	1.07	0.03	1.02–1.13	1.07	0.04	1.01–1.15	1.06	0.05	0.97–1.16
	1955–59	0.98	0.03	0.91–1.04	1.03	0.04	0.97–1.11	1.10	0.05	1.01–1.21	1.14	0.07	1.00–1.29
	1960–64	0.88	0.04	0.80–0.96	0.91	0.05	0.82–1.00	1.09	0.08	0.96–1.25	1.07	0.12	0.86–1.33
	1965–69	0.80	0.05	0.71–0.92	0.84	0.07	0.72–0.98	0.81	0.11	0.62–1.06	0.66	0.18	0.39–1.11
	1970–72	0.94	0.16	0.68–1.30	0.85	0.19	0.55–1.30	1.06	0.38	0.53–2.14	1.73	0.87	0.65–4.62
Maternal age	15–19	1.21	0.03	1.15–1.27	1.17	0.04	1.10–1.24	1.13	0.05	1.04–1.22	1.16	0.07	1.03–1.29
	20–24	1.06	0.02	1.03–1.10	1.06	0.02	1.02–1.10	1.04	0.02	0.99–1.09	1.07	0.03	1.00–1.14
	25–29	1.00			1.00			1.00			1.00
	30–34	1.05	0.02	1.00–1.10	0.98	0.02	0.94–1.02	0.98	0.03	0.93–1.04	1.06	0.04	0.99–1.14
	35–39	1.05	0.04	0.97–1.13	1.01	0.04	0.94–1.09	0.99	0.04	0.91–1.08	0.92	0.05	0.83–1.02
	40+	1.09	0.11	0.89–1.33	1.20	0.12	0.99–1.46	0.80	0.10	0.63–1.02	1.08	0.14	0.84–1.39
Parental social class	I	1.00			1.00			1.00			1.00
	II	1.00	0.05	0.90–1.11	0.98	0.06	0.87–1.09	0.97	0.08	0.82–1.15	0.89	0.13	0.67–1.18
	III	1.22	0.09	1.05–1.41	1.24	0.10	1.06–1.45	1.23	0.14	0.98–1.54	1.05	0.20	0.73–1.52
	IV	0.96	0.05	0.87–1.07	0.94	0.05	0.84–1.05	0.94	0.08	0.80–1.11	0.81	0.12	0.61–1.08
	V–VI	1.18	0.06	1.06–1.31	1.17	0.07	1.05–1.30	1.21	0.10	1.02–1.42	1.08	0.15	0.82–1.43
	Unknown/other	1.25	0.07	1.11–1.40	1.26	0.08	1.11–1.42	1.23	0.11	1.03–1.47	1.10	0.16	0.82–1.47
Number of person months		433,180			359,010			183,505			87,657
Deaths		18,929			17,294			10,720			5,966

Notes: Parental social class categorized according to the Erikson/Goldthorpe/Portocarero scheme. RR indicates relative risk (the reference category is the category for which RR equals one), SE indicates standard errors, and 95% CI indicates 95 per cent confidence intervals.

Source: As for Table 1.