income
Effects of Socio-Economic Status on Offspring Count in Modern Humans
Susanne Huber & Martin Fieder Research Institute of Wildlife Ecology, University of Veterinary Medicine Vienna Department of Anthropology, University of Vienna
content socio-economic status and reproduction i) society sub-sample (university) ii) modern representative sample (Sweden) iii) modern census samples
iv) effects of couple’s education and income on offspring count
human reproduction major limiting factors in evolutionary past: • men: access to fertile females • women: access to resources required for rearing offspring
predictions: •
men: positive association between socio-economic status and reproductive success owing to higher access to fertile women “maximizing number of children”
•
women: positive association between resources and reproductive success “optimizing number of children”
•
Bateman's principle: The sex with the higher investment (women) in reproduction is supposed of being choosier – higher variance in reproduction for the sex (men) with the lower investment
status and reproduction • evidence from traditional and pre-industrial human societies for a positive association between male status/resources and reproductive success. – Turkmen pastoralist (Irons 1979)
– Kipsigis (Borgerhoff Mulder 1988) – Yanomano (Chagnon 1988) – pre-industrial societies (e.g., Mealy 1985)
modern societies • association between status and number of children often found to be null or negative. • explanations so far: – contraception and monogamy (Perussé 1993) – rationality of investing more extensively in fewer children just when the income rises (Becker 1993)
• but data on basis of modern societies may partially be wrong: – sex differences – type of measure of socio-economic status (combined indicators bluring) – reproductive age of study population
i) society sub-sample - university • University of Vienna: anonymised employee database • n = 2693 men, 2073 women • 3 status categories and 6 age groups (30-59 yr) – category 1: academics in leading position with high salary (full professors, associate professors, administrative staff in leading positions) – category 2: academics with intermediate salary and in other than leading positions (other scientists, academic administrative staff in non-leading positions) – category 3: non-academics in non-leading positions with lower salary (non-academic technical and administrative staff)
Fieder, Huber et al. 2005
status and offspring count - men
*** ***
***
*
*
Fieder, Huber et al. 2005
offspring count and gross salary men age
correlation coefficient rs men women
30-34
.13 **
-.16 **
35-39
.07
-.23 **
40-44
.17 ***
-.14 **
45-49
.17 **
-.15 **
50-54
.22 ***
-.10
55-59
.18 ***
-.27 **
Fieder, Huber et al. 2005
status and offspring count - women
***
***
***
Fieder, Huber et al. 2005
offspring count and gross salary women age
correlation coefficient rs men women
30-34
.13 **
-.16 **
35-39
.07
-.23 **
40-44
.17 ***
-.14 **
45-49
.17 **
-.15 **
50-54
.22 ***
-.10
55-59
.18 ***
-.27 **
Fieder, Huber et al. 2005
further results • reproduction after the age of 40 yr – men • status category 1: 10% (extension of reproductive period) • status category 2 and 3: 4%
– women • irrespective of status category < 1%
Fieder, Huber et al. 2005
ii) representative sample – Sweden • representative data from modern Sweden (n = 7000 men, 7000 women aged 45-55 yr; Statistics Sweden) – lifetime number of biological children – marital status (never married vs. married at some point) – income categories • • • •
INC1: 272,500
– education categories • EDUC1: primary and lower secondary education • EDUC2: upper secondary education • EDUC3: tertiary education Fieder & Huber 2007
income/education and offspring count including childless individuals education
Offspring count
income
GLM: men, inc p < 0.001, educ p = 0.538 women, inc p = 0.038, educ p = 0.432
Fieder & Huber 2007
income/education and offspring count excluding childless individuals income
education
GLM: men, inc p = 0.542, educ p = 0.336 women, inc p < 0.001, educ p < 0.05
Fieder & Huber 2007
income/education and childlessness income
education
Fieder & Huber 2007
income/education, marital status and childlessness • association between income/education and marital status
•
association between marital status and childlessness − men: 62.5% of reproducing individuals but only 30% of childless individuals were married at some point − women: 55.0% of reproducing individuals and 40.3% of childless individuals were married at some point Fieder & Huber 2007
IPUMS data marital status/childlessness
summary • positive association between position in an organizational hirarchey on offspring count in men (mostly negative in women) • lengthened reproductive period among high status men • positive association between socio-economic status and offspring count in men attributed mainly to association between income/education and childlessness. • female choice may be major reason for reversed association when including and excluding childless men (Bateman’s principle). – increasing proportion of married men with increasing income/education (further evidence: Nakosteen & Zimmer (1997), Forsberg & Tullberg (1995)). • negative association between education/income and offspring count in women. Fieder & Huber 2007
iii) effects of couple’s education and income on offspring count • how the association between socio-economic status and number of children varies with the source of status and resources, the woman’s education, and her age at reproductive onset (proxied by age at marriage). • IPUMS-US census 1980 (www.ipums.org)*
– women and their spouses aged 45-66yr who are still in their first marriage (n = 504.496). – lifetime number of children of the women. – association between a woman’s offspring count and her own as well as her spouse’s income and education. *Minnesota Population Center. /Integrated Public Use Microdata Series -International: Version 4.0/. Minneapolis: University of Minnesota, 2008).
Huber, Bookstein, Fieder 2010
childlessness and husband’s income Childlessness and Husband's Income 14
% Childless
12 10 8 6 4 2 0 1
2
3
4
5
6
7
8
9
10
11
Husbands Income Percentiles
Huber, Bookstein, Fieder 2010
childlessness and woman’s income Childlesness and Women Income 12
% Chidless
10 8
6 4 2 0 1
2
3
4
5
6
7
8
9
10
Income Percentiles
Huber, Bookstein, Fieder 2010
regression of offspring count regression coefficient incomewoman
-0.027
educationwoman
-0.010
age at marriagewoman
-0.032
incomehusband
+0.005
educationhusband
-0.009
agewoman
-0.005
agehusband
-0.007
R2 = 0.078 Huber, Bookstein, Fieder 2010
woman’s education and mean offspring count educational category
mean offspring count (SD)
n (% of total)
1
4.42 (2.90)
2476 (0.5)
2
4.14 (2.78)
7283 (1.4)
3
3.39 (2.33)
58246 (11.5)
4
3.21 (2.11)
23036 (4.6)
5
3.13 (2.00)
33220 (6.6)
6
2.99 (1.90)
33593 (6.7)
7
2.91 (1.78)
226704 (44.9)
8
2.93 (1.74)
68529 (13.6)
9
2.76 (1.64)
51409 (10.2) Huber, Bookstein, Fieder 2010
mean offspring count by woman’s education and age at marriage
Huber, Bookstein, Fieder 2010
regression of offspring count sorted by woman’s education educational category
regression coefficient incomehusband
incomewoman
educationhusband
1
-0.039
-0.067
-0.046
2
-0.030
-0.061
-0.028
3
-0.020
-0.036
-0.027
4
-0.016
-0.025
-0.016
5
-0.015
-0.021
-0.011
6
-0.011
-0.032
-0.017
7
+0.006
-0.031
-0.005
8
+0.012
-0.022
-0.002
9
+0.019
-0.028
+0.011 Huber, Bookstein, Fieder 2010
association between husband’s income and offspring count by woman’s education and age at marriage
Huber, Bookstein, Fieder 2010
mean offspring count and beta husband’s income
Huber, Bookstein, Fieder 2010
association between woman’s income and offspring count by woman’s education and age at marriage
Huber, Bookstein, Fieder 2010
mean offspring count and beta woman’s income
Huber, Bookstein, Fieder 2010
woman’s education and husband’s income
Huber, Bookstein, Fieder 2010
summary • a woman’s education, income and age at marriage are negatively associated with offspring count and increase the chances of childlessness. • higher husband’s income decreases the chances of childlessness and, among the most highly educated two-thirds of women, increases offspring count. • the association between offspring count and husband's income turns from positive to negative as a woman’s education and age at marriage decline, and family size increases.
• only if family size is small, the association between offspring count and husband’s income is positive. • the association between offspring count and woman’s income is negative regardless of education, age at marriage, and family size. Huber, Bookstein, Fieder 2010
summary II • a woman’s higher education increases her chances for higher own and husband’s income. • on the other hand, a woman’s higher education delays reproductive onset and lowers the number of children.
-> these women appear to adjust offspring number and investment based on family size and resource availability, together with educational level.
-> while the striving for resources seems to be part of modern female reproductive strategy, owing to costs of resource acquisition, especially higher education, it may lead to a decline in birth rate. Huber, Bookstein, Fieder 2010
overall conclusions • mate preferences acquired in our evolutionary past still entail fitness consequences in modern societies. • assumptions drawn from evolutionary theory are still valid in modern societies evolutionary principles may help to explain processes in human societies. • modern humans are well suited to study them from an evolutionary perspective potential implications for other fields. • association between status and reproduction strongly differs between men and women need for data on male offspring number.
Susanne Huber & Martin Fieder Research Institute of Wildlife Ecology, University of Veterinary Medicine Vienna Department of Anthropology, University of Vienna
content socio-economic status and reproduction i) society sub-sample (university) ii) modern representative sample (Sweden) iii) modern census samples
iv) effects of couple’s education and income on offspring count
human reproduction major limiting factors in evolutionary past: • men: access to fertile females • women: access to resources required for rearing offspring
predictions: •
men: positive association between socio-economic status and reproductive success owing to higher access to fertile women “maximizing number of children”
•
women: positive association between resources and reproductive success “optimizing number of children”
•
Bateman's principle: The sex with the higher investment (women) in reproduction is supposed of being choosier – higher variance in reproduction for the sex (men) with the lower investment
status and reproduction • evidence from traditional and pre-industrial human societies for a positive association between male status/resources and reproductive success. – Turkmen pastoralist (Irons 1979)
– Kipsigis (Borgerhoff Mulder 1988) – Yanomano (Chagnon 1988) – pre-industrial societies (e.g., Mealy 1985)
modern societies • association between status and number of children often found to be null or negative. • explanations so far: – contraception and monogamy (Perussé 1993) – rationality of investing more extensively in fewer children just when the income rises (Becker 1993)
• but data on basis of modern societies may partially be wrong: – sex differences – type of measure of socio-economic status (combined indicators bluring) – reproductive age of study population
i) society sub-sample - university • University of Vienna: anonymised employee database • n = 2693 men, 2073 women • 3 status categories and 6 age groups (30-59 yr) – category 1: academics in leading position with high salary (full professors, associate professors, administrative staff in leading positions) – category 2: academics with intermediate salary and in other than leading positions (other scientists, academic administrative staff in non-leading positions) – category 3: non-academics in non-leading positions with lower salary (non-academic technical and administrative staff)
Fieder, Huber et al. 2005
status and offspring count - men
*** ***
***
*
*
Fieder, Huber et al. 2005
offspring count and gross salary men age
correlation coefficient rs men women
30-34
.13 **
-.16 **
35-39
.07
-.23 **
40-44
.17 ***
-.14 **
45-49
.17 **
-.15 **
50-54
.22 ***
-.10
55-59
.18 ***
-.27 **
Fieder, Huber et al. 2005
status and offspring count - women
***
***
***
Fieder, Huber et al. 2005
offspring count and gross salary women age
correlation coefficient rs men women
30-34
.13 **
-.16 **
35-39
.07
-.23 **
40-44
.17 ***
-.14 **
45-49
.17 **
-.15 **
50-54
.22 ***
-.10
55-59
.18 ***
-.27 **
Fieder, Huber et al. 2005
further results • reproduction after the age of 40 yr – men • status category 1: 10% (extension of reproductive period) • status category 2 and 3: 4%
– women • irrespective of status category < 1%
Fieder, Huber et al. 2005
ii) representative sample – Sweden • representative data from modern Sweden (n = 7000 men, 7000 women aged 45-55 yr; Statistics Sweden) – lifetime number of biological children – marital status (never married vs. married at some point) – income categories • • • •
INC1: 272,500
– education categories • EDUC1: primary and lower secondary education • EDUC2: upper secondary education • EDUC3: tertiary education Fieder & Huber 2007
income/education and offspring count including childless individuals education
Offspring count
income
GLM: men, inc p < 0.001, educ p = 0.538 women, inc p = 0.038, educ p = 0.432
Fieder & Huber 2007
income/education and offspring count excluding childless individuals income
education
GLM: men, inc p = 0.542, educ p = 0.336 women, inc p < 0.001, educ p < 0.05
Fieder & Huber 2007
income/education and childlessness income
education
Fieder & Huber 2007
income/education, marital status and childlessness • association between income/education and marital status
•
association between marital status and childlessness − men: 62.5% of reproducing individuals but only 30% of childless individuals were married at some point − women: 55.0% of reproducing individuals and 40.3% of childless individuals were married at some point Fieder & Huber 2007
IPUMS data marital status/childlessness
summary • positive association between position in an organizational hirarchey on offspring count in men (mostly negative in women) • lengthened reproductive period among high status men • positive association between socio-economic status and offspring count in men attributed mainly to association between income/education and childlessness. • female choice may be major reason for reversed association when including and excluding childless men (Bateman’s principle). – increasing proportion of married men with increasing income/education (further evidence: Nakosteen & Zimmer (1997), Forsberg & Tullberg (1995)). • negative association between education/income and offspring count in women. Fieder & Huber 2007
iii) effects of couple’s education and income on offspring count • how the association between socio-economic status and number of children varies with the source of status and resources, the woman’s education, and her age at reproductive onset (proxied by age at marriage). • IPUMS-US census 1980 (www.ipums.org)*
– women and their spouses aged 45-66yr who are still in their first marriage (n = 504.496). – lifetime number of children of the women. – association between a woman’s offspring count and her own as well as her spouse’s income and education. *Minnesota Population Center. /Integrated Public Use Microdata Series -International: Version 4.0/. Minneapolis: University of Minnesota, 2008).
Huber, Bookstein, Fieder 2010
childlessness and husband’s income Childlessness and Husband's Income 14
% Childless
12 10 8 6 4 2 0 1
2
3
4
5
6
7
8
9
10
11
Husbands Income Percentiles
Huber, Bookstein, Fieder 2010
childlessness and woman’s income Childlesness and Women Income 12
% Chidless
10 8
6 4 2 0 1
2
3
4
5
6
7
8
9
10
Income Percentiles
Huber, Bookstein, Fieder 2010
regression of offspring count regression coefficient incomewoman
-0.027
educationwoman
-0.010
age at marriagewoman
-0.032
incomehusband
+0.005
educationhusband
-0.009
agewoman
-0.005
agehusband
-0.007
R2 = 0.078 Huber, Bookstein, Fieder 2010
woman’s education and mean offspring count educational category
mean offspring count (SD)
n (% of total)
1
4.42 (2.90)
2476 (0.5)
2
4.14 (2.78)
7283 (1.4)
3
3.39 (2.33)
58246 (11.5)
4
3.21 (2.11)
23036 (4.6)
5
3.13 (2.00)
33220 (6.6)
6
2.99 (1.90)
33593 (6.7)
7
2.91 (1.78)
226704 (44.9)
8
2.93 (1.74)
68529 (13.6)
9
2.76 (1.64)
51409 (10.2) Huber, Bookstein, Fieder 2010
mean offspring count by woman’s education and age at marriage
Huber, Bookstein, Fieder 2010
regression of offspring count sorted by woman’s education educational category
regression coefficient incomehusband
incomewoman
educationhusband
1
-0.039
-0.067
-0.046
2
-0.030
-0.061
-0.028
3
-0.020
-0.036
-0.027
4
-0.016
-0.025
-0.016
5
-0.015
-0.021
-0.011
6
-0.011
-0.032
-0.017
7
+0.006
-0.031
-0.005
8
+0.012
-0.022
-0.002
9
+0.019
-0.028
+0.011 Huber, Bookstein, Fieder 2010
association between husband’s income and offspring count by woman’s education and age at marriage
Huber, Bookstein, Fieder 2010
mean offspring count and beta husband’s income
Huber, Bookstein, Fieder 2010
association between woman’s income and offspring count by woman’s education and age at marriage
Huber, Bookstein, Fieder 2010
mean offspring count and beta woman’s income
Huber, Bookstein, Fieder 2010
woman’s education and husband’s income
Huber, Bookstein, Fieder 2010
summary • a woman’s education, income and age at marriage are negatively associated with offspring count and increase the chances of childlessness. • higher husband’s income decreases the chances of childlessness and, among the most highly educated two-thirds of women, increases offspring count. • the association between offspring count and husband's income turns from positive to negative as a woman’s education and age at marriage decline, and family size increases.
• only if family size is small, the association between offspring count and husband’s income is positive. • the association between offspring count and woman’s income is negative regardless of education, age at marriage, and family size. Huber, Bookstein, Fieder 2010
summary II • a woman’s higher education increases her chances for higher own and husband’s income. • on the other hand, a woman’s higher education delays reproductive onset and lowers the number of children.
-> these women appear to adjust offspring number and investment based on family size and resource availability, together with educational level.
-> while the striving for resources seems to be part of modern female reproductive strategy, owing to costs of resource acquisition, especially higher education, it may lead to a decline in birth rate. Huber, Bookstein, Fieder 2010
overall conclusions • mate preferences acquired in our evolutionary past still entail fitness consequences in modern societies. • assumptions drawn from evolutionary theory are still valid in modern societies evolutionary principles may help to explain processes in human societies. • modern humans are well suited to study them from an evolutionary perspective potential implications for other fields. • association between status and reproduction strongly differs between men and women need for data on male offspring number.
