Middleborns Disadvantaged? Testing Birth-Order Effects on Fitness in Pre-Industrial Finns
Citation: Faurie C, Russell AF, Lummaa V (
Middleborns Disadvantaged? Testing Birth-Order Effects on Fitness in Pre-Industrial Finns
Charlotte Faurie 0
Andrew F. Russell 0
Virpi Lummaa 0
Rebecca Sear, London School of Economics, United Kingdom
0 1 Department of Animal and Plant Sciences, University of Sheffield , Sheffield , United Kingdom , 2 Institute of Evolutionary Sciences, University of Montpellier, Montpellier, France, 3 Section of Ecology, Department of Biology, University of Turku , Turku , Finland
Parental investment is a limited resource for which offspring compete in order to increase their own survival and reproductive success. However, parents might be selected to influence the outcome of sibling competition through differential investment. While evidence for this is widespread in egg-laying species, whether or not this may also be the case in viviparous species is more difficult to determine. We use pre-industrial Finns as our model system and an equal investment model as our null hypothesis, which predicts that (all else being equal) middleborns should be disadvantaged through competition. We found no overall evidence to suggest that middleborns in a family are disadvantaged in terms of their survival, age at first reproduction or lifetime reproductive success. However, when considering birth-order only among same-sexed siblings, first-, middle- and lastborn sons significantly differed in the number of offspring they were able to rear to adulthood, although there was no similar effect among females. Middleborn sons appeared to produce significantly less offspring than first- or lastborn sons, but they did not significantly differ from lastborn sons in the number of offspring reared to adulthood. Our results thus show that taking sex differences into account is important when modelling birth-order effects. We found clear evidence of firstborn sons being advantaged over other sons in the family, and over firstborn daughters. Therefore, our results suggest that parents invest differentially in their offspring in order to both preferentially favour particular offspring or reduce offspring inequalities arising from sibling competition.
Funding: We are grateful to The Academy of Finland (CF, VL), the CNRS (CF), the Marie Curie Fellowship Scheme (CF) and the Royal Society of London (VL, AFR)
for funding. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
In species with altricial young, parental investment has
profound effects on offspring survival [1,2] and success .
However, parental investment is not unlimited [6,7] and offspring
quality is seldom equal, potentially leading to selection pressures
on parents to invest differentially in their offspring [6,8,9]. The
two most common ways in which offspring are suggested to receive
greater resources is: (a) through direct competition with each
other; and (b) through differential allocation of parental
investment [10,11]. These two alternatives are not mutually exclusive,
and parents might invest so as to either increase or decrease levels
of sibling competition . For example, greater investment in
certain offspring or longer birth intervals can cause inequalities
between siblings. Moreover, parental investment strategies and
offspring outcomes can also depend on the resource availability to
parents or, in humans, in socio-economic factors .
In humans, parents need to divide their limited resources
between several simultaneously dependent offspring [16,17].
While simultaneously raising several offspring of different ages
and developmental stages is typical in humans, such investment
patterns can also occur in other mammals [e.g. meerkats Suricata
suricatta: 18; kangaroos Macropus spp.: 19]. How parents allocate
their investment to each offspring at each given time across their
lifespan is not well-known, and few studies have investigated in any
species how parents should allocate resources among offspring of
different ages [20,21]. Hertwig, Davis and Sulloway showed that if
parents subdivide the resources equally among their offspring at
any given point in time, it yields a cumulative distribution of
investment that is unequal among offspring  (the equity
heuristic, see Figure 1a). The model of Hertwig et al. provides a
useful null hypothesis against which parental investment tactics
can be measured . If parents do not differentially invest in
offspring and if resources do not change over time, then we would
predict that firstborns and lastborns will be more successful than
middleborns which, because of competition, necessarily receive
less care (see Figure 1a). By contrast, in cases where parents
differentially invest resources to particular offspring we would
predict that the pattern of Hertwig et al. will not be upheld. For
example, if parental experience is important for offspring success,
or if parental investment increases as expected residual
reproductive value decreases [23,24], all else being equal, middleborns and
lastborns should be at an advantage (Figure 1b, left hand panel).
At a given time, however, the potential fitness gain that parents
can obtain from firstborns is higher, because: (1) they are more
likely to survive to adulthood, as they have already survived the
first years of life where mortality is the highest; and (2) they are
likely to start reproducing earlier, thereby shortening the
generation time [15,25,26]. If parents favour firstborns from
which fitness benefits are likely to be greatest or middleborns
which are likely to be at a competitive disadvantage, we would
predict greater success among firstborns compared with other
categories or similar success irrespective of birth-order category,
respectively (Figure 1b, middle panel). Finally, if resources
diminish over time , we would expect that lastborns are at a
disadvantage (Figure 1b, right hand panel). We acknowledge that
the predictions from each of the above alternatives are not
mutually exclusive and that other possibilities are equally valid, but
highlight them here as example alternatives to the predictions of
Hertwig et al. (Figure 1a).
Although there are currently no formal tests of such models,
birth-order in humans is known to be associated with various
child attributes. Firstborns generally have a smaller birth weight
. By contrast, later-born children can be out-competed for
resources that are most critical in early childhood. For example,
they are less likely than firstborns to attend infant clinics and to
receive vaccination against childhood diseases , are more
poorly nourished than firstborns [30,31], and in large families,
they run a higher risk of experiencing accidents during early
childhood . Associations between birth order and
educational level have also been found; usually suggesting that
firstborns are advantaged [28,3336]. Consequently, birth-order
has been shown to be linked to a range of traits potentially
associated with offspring fitness, such as their survival ,
wealth inheritance , marriage prospects  and number of
However, the effects of birth order on such results are
commonly contradicting. For example, some studies found that
earlier-born children are at a disadvantage [e.g. 42,43,44], others
found the opposite [e.g. 28,29,45,46], at least one  found a
Ushape mortality risk during the first nine months (elevated risk for
first- and third-borns in Bangladesh) and some have found no
effect at all . These contradictions could be due to different
balances in the relative influences of low birthweight and health in
firstborns, and insufficient parental resources directed towards the
health and safety of later-borns. This balance could further vary
according to environmental conditions experienced by the
different populations studied. Birth-order effects may also be
further influenced in many humans societies by a favoured
inheritance of parental possessions to first-born sons [rule of
primogeniture: 38,40,41,43,48], although there are also societies
where parental wealth is transferred to the youngest son .
Importantly, in most societies, these inheritance rules are mainly
relevant to higher status families where there is wealth to be
While a number of previous studies have thus investigated
birthorder effects (see above), many have failed to control for many of
the potential confounding terms; which might partly account for
contradictory results (see above). It thus would appear critical to
control for all the factors that are known to be linked both to birth
order and to survival or reproductive success. Some studies, for
example, controlled for socio-economic status, but failed to control
for total number of siblings or maternal age [43,49], although this
is critical when investigating birth order effects. Indeed, child
mortality has been found to be negatively related to maternal age
, and lastborns are more likely to be born from an older
mother. Similarly, mothers experiencing high child mortality tend
to have more births, which may make late-birth-order children
appear to have higher than average mortality. In addition, no
study to date has investigated the effects of birth order on a
longterm measure of fitness such as the lifetime reproductive success
(i.e. number of offspring raised to adulthood) that combines
birthorder effects on survival, as well as reproductive rate and the
quality of the offspring produced.
The aim of this study was to use the predictions of the equal
parental investment model (Figure 1a) as a board to investigate
incidences of differential investment on fitness and the key
lifehistory traits underlying fitness in humans. First, we investigate the
consequences of being first-, middle- or last-born on the chances of
surviving to adulthood and reproducing. Second, we analyse the
effects of being first-, middle- or last-born on individual lifetime
reproductive success (LRS) (number of offspring raised to
adulthood over lifetime) as well as the underlying components of
LRS, i.e. lifetime fecundity (number of offspring born) and
offspring survival probability. Finally, we investigate the effects
of being first-, middle- or last-born on the underlying life-history
traits that influence lifetime reproductive success: age at first and
last reproduction and inter-birth intervals. Although the equal
parental investment model (Figure 1a) explicitly considers only
overall birth order (first-, middle- and lastborn child among all
siblings), intra-sex birth order may, at least in some
socioecological contexts, be more relevant to varying patterns of
parental investment. Therefore, we repeated our analyses for
intra-sex birth order categories (first-, middle- and lastborn son
among male offspring only, and first-, middle- and lastborn
daughter among female offspring only, i.e. not necessarily being
first-, middle- and lastborns in the whole family). We also
investigated the effects of being first-, middle-, or lastborn on a
sons probability of becoming a landowner, to test the possibility
that any higher success among firstborn sons may be explained by
gaining access to higher resources at reproductive ages.
We use demographic pedigree records of over 2000 families
from four parishes of rural Finland in the 18th and 19th centuries
. Our demographic data has at least four benefits for
addressing our aims. Firstly, it was collected from church registers
maintained since the 17th century in each parish of the country by
local clergymen, who were obliged by law to submit accurate
records of the survival and reproductive history of all individuals in
their parish area to the state . Migration rates were low and in
most cases the parish migration registers allow the lifetime
reproductive success of dispersers to be determined. Secondly,
the study period ends before the availability of reliable
contraception, freely available healthcare and the associated
transition to low mortality and fertility in Finland, which was
not complete until the mid-20th century . Hence, mortality
and fertility rates are close to natural. Thirdly, the data includes
the social status of each family, which represents differences in
resource availability in terms of nutrition, wealth and workload
between individuals, and has already been shown to influence
reproductive success in our study population [53,54] as well as
other pre-industrial populations [e.g. 14]. Fourth, our data allows
a number of other confounding factors to be controlled, including
geographic (study parish) and temporal (birth cohort) variation in
fertility and mortality, as well as number of siblings, maternal age
and maternal survival.
This study was based on demographic records from historical
Finnish populations. Our data contain three generations of
pedigree data for four geographically separate rural parishes
from the south-west archipelago and from the mainland
(Hiittinen, Kustavi, Rymattyla, and Ikaalinen). During the study
period, these populations depended on farming for their
livelihood, supplemented with fishing in the archipelago areas
of south-western Finland. Farmers could either own their land,
or rent it (tenant farmers). Information on the occupation of
each man, or for women, that of their husband, allowed us to
control for socio-economic status. We contrasted two groups,
according to those owning land versus those either renting or
having no access to land at all. Importantly, these categories
have already been associated with variation in individual
lifehistory traits including fitness among humans living in
preindustrial conditions [46,55]. The mating system was
monogamous. Divorce was forbidden, and so remarriage was permitted
only in the event of spousal death. Child mortality was high,
with only 62% of individuals in our sample surviving to age 15
(the youngest age of first reproduction recorded in this
population). During the study era, inheritance usually favoured
the firstborn son (rule of primogeniture), although inheritance
practices varied in time and place . In most parishes, the
firstborn son inherited the farm (but in the archipelago parishes,
the firstborn daughter could sometimes inherit the farm).
Concerning capital, cattle and personal property, all male
offspring were granted equal shares, while daughters received a
half share . The firstborn son who inherited the farm was
obliged to pay his brothers and sisters their due, but the
compensation was often lower than the value of a share in the
farm and land. The brides dowry consisted of personal property
(e.g. tools, clothes, bed, chest), head of cattle (cows, sheep), and
The study sample includes 2180 men and 2126 women (F1
generation) born between 1732 and 1882 to 716 mothers (P
generation). We recorded full life-history data (survival and
lifetime reproductive events) for all F1 individuals, and followed
their offspring (F2) until age 15. Birth-order from the mothers
point of view was used to build the variable of interest, comprising
three categories: firstborns, middleborns and lastborns. Twins
were excluded from the sample, as were families of less than three
children because the middleborn resource handicap can only be
considered in families of three or more children (Fig 1) . The
resulting sample of generation F1 (3,616 individuals) comprised
577 firstborns (310 males and 267 females), 2,459 middleborns
(1,260 males and 1,199 females), and 577 lastborns (274 males and
303 females). For intra-sex birth order analyses, the male
subsample (1509 men who had at least two brothers) comprised
350 firstborn sons, 809 middleborn sons, and 350 lastborn sons,
and the female subsample (1417 women who had at least two
sisters) comprised 344 firstborn daughters, 729 middleborn
daughters, and 344 lastborn daughters.
Statistical analyses used to address our questions were
conducted using SAS (SAS Institute Inc., release v. 9.1, 2002
2003). In all analyses, the following variables were entered into a
model to control for potentially confounding sources of variation:
sex; mothers age at birth; mothers survival (age at mothers
death); number of siblings; parental socio-economic status
(landowners vs. landless); geographic area (mainland vs.
archipelago); birth cohort (5 categories with 20 years blocks). In all models,
the identity of mothers was fitted as a random term to account for
the use of several offspring within families. Statistically significant
terms at the level of 0.05 were determined. Once the minimal
model was found, birth-order category (our term of interest) was
added and its significance determined. All biologically meaningful
two-way interactions involving birth-order (with sex,
socioeconomic status, number of siblings, and maternal age) were also
tested but only included and reported here if statistically
The proportional response variables (probabilities of surviving
to adulthood and reproducing; survival probability of all children
produced) were analysed with Generalised Linear Mixed Model
(GLMM) where the response term was fitted to a binomial error
structure and a logit link function (using GENMOD function in
SAS). For binary response terms, binomial denominator was fixed
at 1. For the proportion of offspring surviving to adulthood, the
number of surviving offspring (LRS) was considered as the
response term with a variable binomial denominator equal to
the number of offspring born (fecundity). The count response
variables (LRS and fecundity) were analysed with GLMM where
the response term was fitted to a Poisson error structure and a
logarithm link function (using GENMOD function in SAS). The
continuous response variables (age at first and last reproduction,
length of birth intervals) were analysed with linear mixed-effects
models (LME, using MIXED function in SAS) with the response
terms being fitted to a normal error structure and Satterthwaites
formula being used to approximate the denominator degrees of
freedom of each fixed effect .
Probability of surviving to adulthood and reproducing
To determine whether birth-order affected survival probability
to adulthood (age 15), we included all individuals who died before
age 15 or were successfully followed at least until age 15 (3,583
individuals, 99% of our initial sample). For the probability of
reproducing, we restricted our analyses to individuals who
survived to age 15 and who had been successfully followed at
least until the age at which 90% of the individuals in the
population had already finished reproducing, if they were to ever
reproduce in their lifetime (age 50 for men and 44 for women). If
the individual disappeared after this age and had not reproduced
before, he/she was considered as having never reproduced. The
subset of individuals for which reproductive events were known
comprised 843 men and 913 women (77% of individuals who
survived to adulthood). These individuals did not differ from those
excluded from the sample in the proportions of first-, middle- and
lastborns (16%, 68% and 16% in both subsets).
Lifetime reproductive success (LRS)
LRS was measured as the lifetime number of F2 offspring raised
to 15 years. We restricted the sample to include only those
individuals who had been successfully followed at least until the
age at which 90% of the individuals in the population had already
finished reproducing (age 50 for men and 44 for women), for
which LRS is known, and who reproduced at least once (739 men
and 794 women).
Components of LRS
Two life-history traits will govern our measure of LRS: lifetime
fecundity (number of offspring born) and the survivorship of
offspring to adulthood. Lifetime fecundity was considered as a
count response term. The proportion of offspring surviving to age
15 was examined by considering LRS as a response term in a
GLMM with logit link function and a variable binomial
denominator equal to fecundity. For these analyses, we used the
same sample as for LRS.
Underlying life-history traits and probability of
We examined more specific life-history traits involved in LRS:
age at first reproduction, age at last reproduction, and average
inter-birth interval (i.e. length of reproductive life divided by the
number of children). For the analysis of inter-birth interval, we
used the sub-sample of individuals who reproduced at least twice
(694 men and 731 women). Finally, among men who survived to
adulthood, we investigated whether being a first-, middle- or
lastborn son influenced socio-economic status, i.e. whether they
owned a land. The sample of families with at least three sons, for
which both own and parental socio-economic status were known,
comprised 947 men.
Probability of surviving to adulthood and reproducing
In our sample, 63.9% of F1 individuals survived to age 15 (63.3%
of boys and 64.4% of girls, N = 3,583). Survival was positively
associated with mothers (P generation) survival (x21 = 10.43,
p = 0.001), and negatively associated with the number of siblings
(x21 = 18.52, p,0.0001). After controlling for these significant effects,
we found clear, although marginally non-significant, trend for
differences between overall birth-order categories (all siblings of both
sexes considered) in their probability of surviving to adulthood, with
firstborns being disadvantaged, and lastborns having the best
chances of surviving (x22 = 5.76, p = 0.056; Figure 2a). The model
explained 13% of the variance.
When considering intra-sex birth-order, we did not find any
significant differences between first-, middle- and lastborn sons in
their probability of surviving to adulthood (respectively 62%63%,
63%62%, 68%63%; x22 = 2.66, p = 0.3), but we found that
firstborn daughters had a significantly reduced probability of
surviving to adulthood, as compared to their sisters (x22 = 6.48,
p = 0.04; Figure 2b), controlling for total number of siblings
(x21 = 13.56, p = 0.0002) and for mothers survival (x21 = 5.17,
p = 0.02). The model explained 12% of the variance.
Of those F1 individuals who survived to adulthood and were
successfully followed until the end of potential reproductive life,
87.3% reproduced (87.7% of men and 87.0% of women,
N = 1,756). Table 1 provides the descriptive statistics of this data
subset split by sex and parental socio-economic status.
We found no evidence to suggest that probability of reproducing
in a lifetime depended on birth-order category among all siblings
(respectively 89%62%, 87%61%, 88%62%; x22 = 1.39, P = 0.5),
or among brothers only (respectively 87%63%, 86%62%,
88%63%; x22 = 0.23, P = 0.9), or among sisters only (respectively
90%63%, 86%62%, 87%63%; x22 = 1.42, P = 0.5).
Lifetime reproductive success (LRS)
Among F1 individuals who survived to age 15, who were
successfully followed until the end of their potential reproductive
life, and who reproduced at least once, the average LRS was
3.362.2 s.d., and the maximum was 11. LRS differed between
geographical areas (x21 = 13.30, p = 0.0003), and between cohorts
(x24 = 20.36, p = 0.0004). In this subset of individuals who did
reproduce, men had more children than women (x21 = 6.96,
p = 0.008). LRS was positively associated with P mothers survival
(x21 = 10.90, p = 0.001). After controlling for these effects, we
found no evidence to suggest that LRS depended on birth-order
category among all siblings (x22 = 0.83, p = 0.7).
When considering intra-sex birth-order categories, we found
that first-, middle- and lastborn daughters did not differ for LRS
(2.960.2, 3.360.1, 3.360.2; x22 = 2.76, p = 0.3). However, first-,
middle- and lastborn sons did differ (x22 = 7.41, p = 0.02),
controlling for total number of siblings (x21 = 8.48, p = 0.004):
firstborn sons were strongly advantaged (Figure 3a). The model
explained 8.6% of the variance.
(SES: parental socio-economic status; N: sample size; % Rep: probability of reproducing; AFR: age at first reproduction; ALR: age at last reproduction; L. Fec.: lifetime
fecundity; LRS: lifetime reproductive success).
Components of LRS
Among F1 individuals who survived to age 15, who were
successfully followed until the end of their potential reproductive
life, and who reproduced at least once, the average lifetime
fecundity (number of children born) was in our sample 5.463.0
s.d. (N = 1,533) and the maximum was 16. Lifetime fecundity
differed between geographical areas (x21 = 39.69, p,0.0001) and
between birth-cohorts (x24 = 9.29, p = 0.05). In this subset of
individuals who did reproduce, men had more children than
women (x21 = 7.44, p = 0.006). Lifetime fecundity was also
positively associated with P mothers survival (x21 = 12.86,
p = 0.0003) and with the number of siblings (x21 = 4.26,
p = 0.04). After controlling for these effects, we found no evidence
to suggest that lifetime fecundity was associated with overall
birthorder category (5.560.2, 5.460.1, 5.760.2; x22 = 0.88, p = 0.6).
However, when investigating the effect of intra-sex birth-order
categories, we found that firstborn sons produced significantly
more offspring than their brothers and that middleborn sons
produced the smallest number of offspring (x22 = 12.91, p = 0.002;
Figure 3b), controlling for area (x21 = 8.97, p = 0.003), total
number of siblings (x21 = 15.15, p,0.0001) and mothers survival
(x21 = 6.54, p = 0.01). This model explained 17% of the variance.
Additionally, we found that when each adult sons own
socioeconomic status was included in the model (x21 = 2.64, p = 0.1),
this effect of intra-sex birth-order category on their fecundity was
still significant (x22 = 12.28, p = 0.002), suggesting that that the
favoured position of first-borns sons as the inheritors of the
parental land was not the sole reason for their higher lifetime
fecundity. No differences were found between sisters birth order
categories for fecundity (respectively 4.960.2, 5.260.2, 5.260.2;
x22 = 1.33, p = 0.5).
On average, 62% of offspring (F2) survived to adulthood
(N = 1,533). This percentage differed between geographical areas
(x21 = 6.08, p = 0.01), between birth-cohorts (x24 = 19.26,
p = 0.0007), and increased with socio-economic status
(x21 = 3.13, p = 0.07). In addition, it was negatively associated
with the number of siblings (x21 = 11.96, p = 0.0005). After
controlling for these effects, we found no evidence to suggest that
the proportion of an individuals offspring surviving to adulthood
differed according to his/her overall birth-order categories in a
family (respectively 63%62%, 62%61%, 61%62%; x22 = 1.01,
p = 0.6) or intra-sex birth-order categories (among brothers:
respectively 60%62%, 61%62%, 61%62%; x22 = 0.16, p = 0.9;
among sisters: respectively 58%62%, 62%62%, 62%62%;
x22 = 1.80, p = 0.4).
Underlying life-history traits and probability of
The average age at first reproduction among F1 individuals
differed between men and women (F1,1439 = 48.88, p,0.0001; see
Table 1). Age at first reproduction also differed between
geographic areas (F1,1412 = 8.13, p = 0.005) and was positively
associated with maternal age (P generation) (F1,1471 = 5.98,
p = 0.01). After controlling for these effects, we found that age at
first reproduction did not significantly depend on overall
birthorder category (respectively 27.560.4, 27.460.2, 26.460.4;
F2,1415 = 2.60, p = 0.074). However, when considering intra-sex
birth-order categories, we did find that firstborn sons began
reproduction on average almost two years earlier than their
brothers (F2,560 = 6.54, p = 0.002; Figure 4a); in contrast, age at
first reproduction did not significantly differ among sisters
(respectively 26.260.4, 26.460.3, 25.960.4; F2,564 = 0.78,
p = 0.5).
The average age at last reproduction differed between men and
women (F1,1462 = 54.29, p,0.0001; see Table 1). Age at last
reproduction also differed between geographic areas
(F1,380 = 18.22, p,0.0001) and was positively associated with
mothers survival (F1,472 = 11.14, p = 0.0009). After controlling for
these effects, we found no evidence to suggest that age at last
reproduction depended on overall birth-order categories
(respectively 38.660.5, 39.460.2, 39.160.5; F2,1418 = 1.27, p = 0.3) or
intra-sex birth-order categories (among brothers: respectively
40.160.7, 40.560.5, 41.660.7; F2,563 = 1.23, p = 0.3; among
sisters: respectively 37.160.5, 38.060.4, 37.460.5; F2,574 = 1.24,
p = 0.3).
Finally, among individuals who reproduced at least twice,
average inter-birth interval (i.e. the length of reproductive life
divided by the number of children produced) differed between
birth cohorts (F4,887 = 2.83, p = 0.02) and depended on parental
socio-economic status (F1,448 = 6.22, p = 0.01), but we found no
difference between overall birth-order categories in a family
(respectively 2.2760.06, 2.3260.04, 2.3260.07; F2,1348 = 0.27,
p = 0.8 or between intra-sex birth-order categories (among
brothers: respectively 2.1860.09, 2.3360.06, 2.3760.09;
F2,500 = 1.38, p = 0.3; among sisters: respectively 2.3160.07,
2.3060.06, 2.2960.08; F2,530 = 0.02, p = 0.9).
Among men, own socio-economic status as an adult was
strongly positively associated with parental socio-economic status
(x21 = 131.08, p,0.0001). After controlling for this effect, we found
that intra-sex birth-order category also had a significant effect,
with firstborn sons being much more likely than their brothers to
become landowners (x22 = 26.47, p,0.0001; Figure 4b). This
model explained 23% of the variance.
Parental investment has profound effects on offspring success
[3,4,60]. Given that parents accrue differential fitness benefits
from their offspring, parental investment (and particularly
differential investment) tactics are currently an expanding subject
of research in evolutionary ecology. Hertwig, Davis and Sulloway
provided a useful first step by offering a potential null hypothesis
against which observations of birth order effects on offspring
success can be compared . Our study investigated, in humans,
differences between first-, middle- and lastborns using lifetime
fitness measures, while simultaneously controlling for maternal
age, total number of siblings and socio-economic status.
Hertwig et al. posit that all else being equal, differences in the
amount and quality of parental investment received by offspring
can arise even in the presence of an equity motive on the parents
side: in families of three or more children, middleborns will receive
fewer resources due to competition with younger and elder siblings
. When considering overall birth-order in a family, we found
no evidence for this idea despite using a large, detailed and
representative dataset of over 2000 families living in pre-industrial
Finland. Middleborn offspring did not have reduced probabilities
of surviving to adulthood or reproducing, or reduced lifetime
reproductive success. Indeed, they neither had reduced fecundity
nor gave rise to offspring with reduced survivorship. Given the fact
that (all else being equal) middleborns should be disadvantaged in
humans and that yet we found little supporting evidence for this, it
is likely that parents increase investment in middleborn offspring.
However, it should be noted that when considering birth-order
effects among males of the sibship, we found that middleborn sons
gave birth to fewer offspring, and showed a tendency to have a
lower LRS than their brothers. This was partly because they began
reproduction on average one year later than their brothers.
Middleborn daughters, within all the daughters of a sibship, did
not suffer similar costs as middleborn sons.
We found some evidence that parental inexperience negatively
influenced offspring survival to adulthood. Children who had the
lowest chances of surviving were firstborns. This was especially
true for firstborn daughters, who were less likely to survive than
their sisters. Firstborns are often born smaller in humans , and
birthweight is related to health in adulthood . That we did not
find reduced fitness among firstborns suggests some degree of
compensatory investment following birth either directly through
(grand)parents or through sibling competition.
In fact, modelling resource allocation among different-aged
offspring indicates that, in nearly all circumstances, the
evolutionary stable strategy is to bias investment toward older offspring
. In line with this idea, primogeniture (first offspring
inheritance) is widespread in human populations [for a review,
see 38], and firstborns tend to end up in an advantaged position
during upbringing with regard to both health [29,62] and
educational achievement [34,63]. In accordance with this, our
results showed that firstborn sons were those who gave birth to the
largest numbers of offspring, and this is presumably because they
began reproduction earlier. This is likely mostly explained by their
higher access to resources: firstborn sons were more likely to
become landowners, and landownership is known to be related to
reduced ages at first marriage and reproduction in this population
. These findings suggest that parental resources diminish over
time, in contrast to the model of Hertwig et al.
A further limitation of the Hertwig et al. model is that it fails to
consider the possibility that older brothers and older sisters are
likely to have, at least in some socio-ecological settings, a different
influence on their younger siblings fitness. For example, in rural
Gambia, children with any living sisters who were at least 10 years
older than the child had lower mortality rates, whereas the
presence of older brothers did not improve survival chances .
Among the Gabbra pastoralists, whereas the number of older
brothers had a negative effect on mens reproductive success (as a
result of smaller initial wealth and later age at marriage), the total
number of sisters had a significant positive effect . Similarly,
among the Kipsigis of Kenya, mens reproductive success (number
of offspring surviving to 5 years) decreased with the number of
brothers and increased with the number of sisters . Low 
showed that the number of elder brothers negatively correlated
with mens number of offspring in 19th century Sweden. Finally,
using the same dataset as the present study, Rickard et al. [65,66]
have shown that offspring born after a brother have lower lifetime
reproductive success than those born after a sister. In line with
these findings, our analyses showed that intra-sex birth-order
affected fitness correlates in a different way among male and
female siblings. Firstly, being a firstborn son provided a
reproductive advantage, even though a firstborn son can be a
middleborn in the family as a whole. This is obviously the
consequence of wealth inheritance practices, as illustrated by the
fact that firstborn sons were more likely to own a land than other
men. We did not find evidence for such an advantage of firstborn
daughters. While firstborn men breed with a richer resource-base
than their younger brothers, this may not be true of females who
marry out of the family. Secondly, the probability of surviving to
adulthood was on average lower for firstborn daughters than for
other girls, even though this group comprises both firstborns and
middleborns in the family as a whole. The fact that, in contrast,
survival was not significantly reduced for firstborn sons could be
due to an increased effort made by parents to protect the life of
their first son, especially in his early years, because at that time he
is still the only son they have, and therefore the only potential
heritor of their wealth.
Finally, the Hertwig et al. model fails to take into account
differences in the timing of resource acquisition. In the equal
investment model, the resources received for the first and last
birth rank are identical in quantity, but not in timing. For
example, consideration of the resources received during the first
two years of first- and lastborns in a family of four children
(Figure 1a), illustrates this point. This difference in the timing of
parental resource availability can be significant, for early
conditions can dictate a whole host of future characteristics
In conclusion, we have shown that birth-order category in the
sibship as a whole has little influence on fitness. By contrast, we
found that, among male offspring in this population, firstborn sons
had a significantly higher fitness than their brothers, and
middleborn sons tended to have the lowest fitness. These results
contrast with predictions of the equal investment model ,
indicating that mothers preferentially invest in certain offspring.
Future models which expand the equal investment model to
include sex differences and diminishing parental resources will be
particularly valuable in providing predictive frameworks to studies
of parental investment in viviparous animals. Our results suggest
that resources acquisition by offspring in humans is not simply a
consequence of sibling competition, but that parents actively
provide extra resources to those offspring which are likely to be
under the greatest competition, or which are the greatest fitness
We thank Aino Siitonen, Kimmo Pokkinen and Timo Verho for collection
of the demographic dataset, Samuli Helle, Mirkka Lahdenpera, Jenni
Pettay and Ian Rickard for assistance during the analyses, and Violaine
Llaurens and three anonymous referees for useful comments on the
manuscript. Contribution 2009-056 of Institut des Sciences de lEvolution
de Montpellier (UMR CNRS 5554).
Conceived and designed the experiments: CF VL. Analyzed the data: CF.
Wrote the paper: CF AFR VL.
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