Does Investment in the Sexes Differ When Fathers Are Absent?

15 Pages·2008·0.25 MB·English

HumNat(2008)19:263–276
DOI10.1007/s12110-008-9044-2
Does Investment in the Sexes Differ When Fathers
Are Absent?
Sex-biased Infant Survival and Child Growth in Rural Ethiopia
Mhairi A. Gibson
Publishedonline:22July2008
#SpringerScience+BusinessMedia,LLC2008
Abstract This study examines child survival and growth in a patrilineal Ethiopian
community as a function of father absence and sex. In line with evolutionary
predictionsforsex-biasedparentalinvestment,theabsenceofafatherandassociated
constraints on household resources is more detrimental for sons’ than daughters’
survivalininfancy.Fatherabsencedoublesason’sriskofdyingininfancybuthasa
positiveinfluenceonthewell-beingoffemalemembersofthehousehold,improving
daughter survival, growth, and maternal nutritional status. Lack of paternal
investment may be compensated for by other matrilateral kin through increased
reciprocity between mother, daughter, and sister.
Keywords Parentalinvestment.Sexpreferences.Fatherabsence.Kinnetworks.
Trivers-Willard.ArsiOromo(Ethiopia)
Evolutionary life-history theory provides an explanatory framework that outlines
why and how women invest more in offspring than men, owing to a slower rate of
reproduction (Trivers 1972). A father’s investment in offspring is seen to be more
variable than maternal investment and is dependent on his degree of paternity
confidence (Alexander 1974; Anderson et al. 2007), a consequence of internal
female fertilization and concealed ovulation, which allows only a mother to be
certainofhergeneticrelatednesstooffspring(Strassman1981).Further,tooffsetthe
costs of their higher investment, women seek additional support from other
genetically related (maternal) kin (Hrdy 2005; Sear and Mace 2008), who are
thereby able to improve their inclusive fitness (Hamilton 1964).
The importance of mothers (Pavard et al. 2004; Reher and Gonzáles-Quiñones
2003) and extended female relatives for successful child-rearing is well-established
in the evolutionary life-history literature (Hawkes et al. 1997; Leonetti et al. 2005;
M.A.Gibson(*)
DepartmentofArchaeology&Anthropology,UniversityofBristol,43WoodlandRd.,BristolBS8
1UU,UK
e-mail:[email protected]
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Sear 2006; Sear et al. 2000, 2002); however, the significance of fathering is less
conclusive. Less than third of studies identify a positive effect of fathers on child
survival (Sear and Mace 2008), despite providing protection and support during the
child’s development (Draper and Harpending 1982), and at times contributing
equivalent amounts of direct support as extended female relatives (Huber and
Breedlove 2007). This paper considers the role of paternal investment in an
Ethiopian Oromo agropastoral community.
Do Fathers and Mothers Invest Differentially by Sex?
Anthropologists have traditionally argued that residential and descent patterns
underlie sex preferences in children since they influence the types of kin who are
abletocooperate.Inmatrilineal,matrilocalsocieties,awoman,herdaughter,andher
female relatives are expected to support one another (e.g., the Chewa: Sear 2006;
Holden et al. 2003; and the Khasi: Leonetti et al. 2004); in patrilineal, patrilocal
societies there is likely to be greater cooperation between a man, his son, and his
male relatives (e.g., rural Greece: Pashos 2000).
StudiesfromtheUSandEuropeanpopulationshavefocusedongenderdivisions
insex preferences, presentingfathersas being more involved inparentingsons than
daughters. Child custody is more commonly bestowed on fathers than mothers, and
fathers bestow more child support on their sons than on their daughters (Raley and
Bianchi2006).Fatherswithsonsworklongerhoursforhigherwages(Lundbergand
Rose 2002), while mothers preferentially provide support (usually emotional) to
their daughters (Suitor and Pillemer 2006). However, such father-son, mother-
daughter dyads are not consistent across cultures. In some societies mothers invest
more heavily in sons while fathers invest more in daughters (Cronk 1991; Haddad
and Hoddinott 1994; Quisumbing and Maluccio 2003). Evidence from African
populations, including Ethiopia, suggest that a father’s assets improve, whereas a
mother’s assets have a negative effect on, daughters’ educational outcomes
(Quisumbing and Maluccio 2003).
Evolutionary Theories on Sex-biased Parental Investment
Early evolutionary biologists recognized that under certain well-defined conditions,
astrategy ofsex-biased childinvestmentcouldbeevolutionarily adaptive (Maynard
Smith 1980; Trivers 1972; Trivers and Willard 1973). Trivers and Willard (1973)
proposed that in polygynous species, such as humans, under favorable conditions
parents are expected to bias investment toward male offspring, whereas under poor
conditions they should bias toward females. This finding is based on a number of
assumptions—that males have greater variance in reproductive success, which is
moregreatlyinfluencedbyparentalinvestment/condition,andthatparentsbiastheir
investment in favor of the sex that yields the greater fitness returns per unit of
investment. Following from this,Godoyetal.(2006)suggestthat sexdifferences in
resourceavailabilitymayunderliesexbiasesinmaternalandpaternalinvestment;in
otherwords,forparentsitwouldbeevolutionarilyadaptiveforthesexsufferingthe
HumNat(2008)19:263–276 265
greatest resource constraint to favor daughters, while the sex with the least resource
constraint should favor sons.
TheoriginalTrivers-Willardmodelwasproposedtoexplainvariationinsexratio
at birth, thus identifying variation in prenatal investment. This relationship has
previously been demonstrated for the Arsi Oromo, among whom women with
improved nutritional status are more likely to give birth to sons, while those in
poorer condition give birth to daughters (Gibson and Mace 2003). The Trivers-
Willardmodelofsex-biasedinvestmenthasalsobeenappliedtopostnatalchild-care
behaviors, albeit with mixed results (Keller et al. 2001). Socioeconomic status is
routinely used as a proxy for parental condition, with wealthy families biasing
investment toward boys and poorest families showing the greatest consideration for
girls in terms of education outcomes (Borgerhoff Mulder 1998), breast-feeding
practices (Bereczkei and Dunbar 1997), and healthcare (Cronk 1989).
Other evolutionary anthropologists have emphasized the potential for both
cooperation and conflict between same-sex relatives (local enhancement/local
competition models, as reviewed in Sieff 1990). Sex-specific costs and benefits are
identified as influencing parental investment: in other words, where there is a
disparity in offspring productivity (Quinlan et al. 2005), offspring contribution to
parents’ fertility—for example, assistance with child care (Bereczkei and Dunbar
1997)—and/orsame-sexcompetitionforresources,suchasinheritablewealth(Mace
1996; Borgerhoff Mulder 1998), parents are expected to favor the sex with the
greatest fitness payoff per unit of their investment.
Research Questions
Thispaperexploreshowtheabsenceofafatherinfluenceschildinvestmentpatternsin
apatrilineal,male-dominatedsocietyinruralEthiopia,theArsiOromo.Themainaim
istotestapredictionforsex-biasedpostnatalparentalinvestmentbasedontheTrivers-
Willardmodel:thatsonsareprioritizedwherefathersarepresentandsocioeconomic
conditions are good, while daughters are advantaged under poorer conditions where
fathers are absent. In testing this, the paper addresses the following questions: Does
fatherabsenceresultinhouseholdresourcerestriction?Doesthisalterchildsurvivalin
earlyinfancyand/orlatergrowthduringchildhood?Isfatherabsencemoredetrimental
forsonsorfordaughtersduringthesetwoperiodsofchilddevelopment?
Study Site
The Arsi Oromo agropastoralists inhabit villages in the Rift Valley area of southern
Ethiopia.Formerlynomadiccattle-herders,theybegantosettleandcultivatelow-lying
areas during the mid twentieth century (Trimingham 1965). Today the Arsi Oromo
subsistence is predominantly based on crop production, with maize and sorghum
being the main crop types in the low-lying areas; however, cattle herding is still
prevalent.Playinga central roleinOromocultural life,cattle aretheprimary unitof
exchange at marriage (via bridewealth; Baxter 1996) and form the basic measure of
wealth.Followingaprogramoflandredistributionintroducedinthe1980sand1990s,
266 HumNat(2008)19:263–276
and inthe absence ofimprovementsinagrarianeconomy orother income-generating
activities, the degree of socioeconomic differentiation between households remains
limited (Gibson 2002). Furthermore, the region has suffered periods of acute food
shortagessincethe1990s,resultinginhighlevelsofchildhoodmortality.Asurveyin
2000 indicated that almost 25% of children die before the age of 5 (Gibson 2002).
Many villages remain without transport or communication links and are more than
four hours’ walking distance from the nearest market town, school, or health clinic.
Themajorityoftheruraladultpopulation(75%)lackformalschooling(Gibson2002).
Abban durbaa maqaa hinqabu [The father of a girl has no name]. —Traditional
Oromo saying
TheArsiOromoexpressastrongculturalpreferenceforsonsoverdaughters,theideal
family being composed of twice as many sons as daughters (Gibson 2002). Son
preference, expressed by both parents, is said to relate to the system of village/clan
exogamy, whereby daughters leave their natal village/clan to join the husband’s
lineageatmarriage.Preferentialchild-carepracticestowardmalechildrenareevident
in the study community with regard to healthcare, education, and breast-feeding
practices(Gibson2002),reflectingnationalpreferences(ShortandKiros2002).Sex
ratioatbirthinthispopulationofArsiOromois1.07(withintheexpectedrangefor
mammals) buthasbeenshowntovaryaccording tomaternal condition(Gibsonand
Mace 2003), meeting the requirements of the original Trivers-Willard hypothesis
regarding sex biases in prenatal investment (Trivers and Willard 1973).
Marriage and Remarriage
Marriages are arranged between clans in differing villages, permitting important
alliances to be forged between communities with differential access to resources,
such as water and grazing land (Terefe 2000). In rural areas, marriage is universal,
but it is common for husband and wife to have a 10-year discrepancy in age. Mean
ageatfirstmarriageformarriedwomenis15.95±4.15,andformarriedmen,23.12±
4.15(Gibson2002).Sincehouseholdwealthisinheritedthroughthemaleline,most
newlyweds reside in the groom’s village (they are patrilocal), at which point the
woman becomes entirely dependant on her husband’s land and cattle (Gibson and
Mace 2007). Typically married women in this region find themselves relocating to
villages more than one hour’s walking distance from their natal village, permitting
intermittentcontactwiththeirownrelatives(maternalkin)(GibsonandMace2005).
Widowhood and Father Absence
Among the Arsi Oromo, divorce or separation is extremely rare: <1% of marriages
end in divorce/separation, but levirate marriage (to a brother’s widow) is common.
Early widowhood is a consequence not only of the age difference between spouses
but also the armed conflicts occurring in the region since the mid twentieth century.
In a household census on this study population, 8.4% of women in the villages
reported that they had been married more than once (Gibson 2002) and 10% of
householdswithchildrenwerewithoutabiologicalfatherowingtodeathordivorce.
HumNat(2008)19:263–276 267
Followingwidowhood,remarriage toa husband’s brotherisculturally proscribed;
however,itisgenerallyconsideredtoresultinlessfavorablelivingconditionsforall
thoseconcerned.Inenteringapolygynousuniontoherhusband’sbrother,usuallyasa
low-rankingco-wifewithoutbridewealthpayments,awomanandherchildrensuffer
social,nutritional,andeconomichardshipsassociatedwithhouseholdresourcedilution
(GibsonandMace2007).Womenwhoarenotsupportedbytheirhusband’srelatives
return to their natal villages and seek additional support for their children from their
own (matrilateral) relatives. In this case, male children are obliged to return to live
with their father’s (patrilateral) relatives upon reaching adulthood (15 years of age).
Conditions within such father-absent households are considered to be more
socioeconomically disadvantaged than other households where the father is present.
Awidow’s situation is described as “very weak. . . . Even if she is married to her
husband’sbrother,hedoesn’tlookafterherlikehisownwife....Itisbetterforher
children if she returns to her parents’ village” (as described by a local informant).
Among the Arsi Oromo, father absence is an indicator of both household
socioeconomic status and proximity to maternal (matrilateral) kin.
Methods
Data Collection
Since 1999 the author has collected demographic and anthropological data from
eight villages in the lowland Arsi region in southern Ethiopia. Demographic census
surveys were undertaken in these villages between 1999 and 2003, during which
retrospective reproductive histories were collected from 1,600 reproductive-age
women (15–50 years). This included a detailed event history calendar that recorded
themonthlytimingofbirths,deaths,andmaritaleventsoverthesixyearspreceding
theinterview.Eachlifehistoryeventwasdatedtotheyearandtothemonthusinga
calendar marked with local significant events. In addition, a household survey
recorded detailed information on the household composition (e.g., age, sex,
educational status) but also household livelihoods (e.g., herd size) and marriage
and residence patterns (e.g., marriage start and end dates, age at marriage, dates of
movement) of all household members.
Over the same period, a sample of 500 women (15–50 years) and 700 children
(<16 years) was included in an anthropometric survey across the villages. Measures
ofheightandweightwereobtainedbytheauthorusinganthropometerandelectronic
scales following the guidelines set out by Lohman et al. (1988). The median age of
thesampleofchildrenmeasuredduringthesurveywasfiveyearsandsevenmonths.
Thesampleincludedanequal numberofbothsexes (50.4%females,49.6% males).
Analyses
The longitudinal demographic data were analyzed using multivariate hazards
regression analysis to assess the effects of father absence on monthly risk of infant
death. Risk of mortality was modelled up to the first year of life, since the greatest
268 HumNat(2008)19:263–276
riskofachilddyingwasknowntooccurduringthefirstmonthsoflife(Gibsonand
Mace 2006). Hazards regression analysis is a powerful statistical tool for isolating
precisedemographiceffects,andunlikeotherstandardformsofregressionanalysisit
can deal with both censored data and time-series data (Allison 1984; Yamaguchi
1991). It can be used to investigate the effect of several factors, such as sex and
fatherabsence,onthetimingofaspecifiedevent(e.g.,infantdeath),withoutmaking
any assumption about the timing of an event in the future. In this case, Cox’s
proportional hazards regression was used to estimate the multivariate models,
providing odds ratios and 95% confidence intervals for monthly risk of death under
1 year of age.
Additional cross-sectional anthropometric analyses for children were undertaken,
with childhood growth up to adulthood (1–15 years) used as a measure of post-
weaning child-care practices. Multivariate general linear models (GLM) were
employed to assess the partial effects of child’s age, sex, and height, as well as a
covariate for father absence, on child weight in kilograms at the date of survey. In
this case, general linear models provide adjusted means and standard errors for
growth under the age of fifteen, after controlling for other variables.
To determine whether father absence is associated with household resource
restriction in this population, chi-square and t-tests were undertaken on categorical
(% without cattle) and continuous (herd size, maternal age, maternal weight)
variables, respectively. Herd size was considered the best measure for wealth since
land had been redistributed equally following land reform in the 1990s.
Covariates
To assess the impact of father presence on the outcome variables (infant death and
child growth), a dichotomous variable recording father presence was created and
includedinmodels.Thiscovariate,FatherAbsence,identifiedallcasesinwhichthe
mother of the child was widowed, divorced, or separated from the biological father
of the child at any point during the observation period (from birth to survey date
within the 5-year calendar). Other covariates known to influence infant mortality
wereincludedinthehazardsregressionmodel:lengthofexposure(timesincebirth),
sex of child, year of birth, and family size (as of the survey date). Separate models
[not presented here] were run to explore interaction effects between each of the
independentvariables. Theonlysignificant interactionterm,between father absence
and sex of the child, was included in the final models.
In all the analyses, although current-status measures (herd size, family size, and
nutritional indices) refer to information collected in the survey in 2003–2004, the
maximum amount of time that had elapsed between any demographic events
included in the analyses and the survey date was 5 years.
Thefinal datasets werelimited tothree children permother/household,tocontrol
forhierarchicalstructuresofthedatarelatingtofamilycharacteristics(geneticand/or
environmental similarities within families). A total of 1,240 women, one per
household, contributed 3,720 births in the final hazards regression analyses. There
was anequal sample of males andfemales (51% males and49% females).Totalsof
571 children (1–15 years) and 405 women were included in the final nutritional
analyses. SPSS version 14.0 was used to perform all the statistical analyses.
HumNat(2008)19:263–276 269
Table1 Sociodemographiccharacteristicsandnutritionalstatusofmothers
Householdwealthindicators Mother Maternalweight
Father n Herdsize %Withoutcows n Maternalage kg(mean±SD)
(mean±SD) (mean±SD)
Absent 134 2.20±2.7 32.1 41 31.25±6.7 51.29±6.53
Present 1,105 2.93±3.3 21.2 365 31.88±9.29 48.92±5.81
t-testa chi-squarea t-testb chi-squarea
aSignificant
bNotsignificant
Results
Table 1 presents the socioeconomic characteristics of households in the villages in
relationtofatherabsence.Thefirstpartofthetablerevealsthathouseholdswithouta
fatherarepoorerthanthoseinwhichthefatherispresent.Themeanhouseholdherd
sizeis2.99±3.42,buttherangeextendsfromzeroto41.Fatherabsenceisassociated
with a smaller householdcattleherdsize relativetohouseholdsinwhicha father is
present [t=2.87, df=1238, p=0.005]. More than 20% of all households have no
cattle; however, this varies according to father absence. A higher percentage of
households without a father have no cattle (32.1%), compared with households in
which a father is present (21.2%) [χ2=13.47, df=1, p=0.004].
The second part of Table 1 describes the age and weight in a sample of ever-
married women (15–50 years) in relation to father absence. While there are no age
differencesbetweenwomen[t=0.504,df=404,p=0.616],therearecleardifferences
intheirnutritionalstatus.Despitebeingpooreroverall,widowedordivorcedwomen
areonaverage2.4kgheavierthanthosewhosechildren’sfatherispresent[t=−2.30,
df=404, p=0.022]. The demonstrated improvement in women’s nutritional status
following marital dissolution may relate to increased proximity and assistance from
Table2 Relativemonthlyrisk(oddsratiosandβparameterestimates)ofinfantdeath,under12months
ofage(n=3,720)
Parameter n Oddsratio 95%CI β±SE p
Age – 0.996 0.987–1.005 −0.004±0.004 0.379
Sex
Male 1,881 1.306 1.053–1.62 0.267±0.11 0.015
Female 1,839 1.000 – – –
Yearofbirth – 0.950 0.893–1.011 −0.051±0.032 0.108
Familysize – 1.011 0.987–1.036 0.011±0.012 0.361
Fatherabsence
Absent 373 0.556 0.273–1.133 −0.587±0.363 0.106
Present 3,347 1.000 – – –
Sex×Fatherabsence
Maleswithfatherabsent 2.369 1.06–5.296 0.862±0.411 0.036
Deaths(n=378)
Births(n=3,720)
Includes only three births per woman. Bold type indicates child’s relative risk of mortality was
significantlydifferentfromthereferencecategory(OR=1.00)
270 HumNat(2008)19:263–276
Fig.1 Kaplan-Meierplotshow-
ingsurvivalcurvebyfather
absenceforsons
theirown(matrilateral)relatives,sincewomengenerallyreturntotheirnatalvillages
following widowhood or divorce.
Infant Mortality
A multivariate cox regression model for assessing the risk factors associated with
infantdeath(monthlyriskunder12months)ispresentedinTable2.Overall,10.1%
(378/3720) of births died before 12 months; 18.2% (678/3720) had not yet reached
12 months of age by the date of survey and thus were censored.
In the multivariate analyses, covariates for age, year of birth, and family size are
not significant predictors of monthly risk of death. Sex strongly influences an
infant’s risk of dying; male infants have a 30% increased risk of death per month
thanfemales[OR=1.306,β=0.267±0.11,p=0.015].Fatherabsenceonlybecomesa
statisticalpredictorofdeathwhenaninteractiontermwithsexisaddedtothemodel
Fig.2 Kaplan-Meierplotshow-
ingsurvivalcurvebyfather
absencefordaughters
HumNat(2008)19:263–276 271
Table3 Estimatedmarginalmeansofchildweight-for-heightinkilograms(0–15years;±SE)foreach
sexbyfatherabsence(n=571)
n Weightforheightandage,2003
Marginalmean±SE β±SE p
Sex
Male 277 14.88±0.45 – –
Female 294 16.65±0.40 3.49±1.19 0.004
Fatherabsence
Absent 16 16.63±0.59 5.26±0.90 1.00
Present 555 14.91±0.10 – –
Sex×Fatherabsence
Absent×Male 7 14.88±0.89 – –
Absent×Female 8 18.37±0.80 3.44±1.21 0.005
Present×Male 270 14.88±0.14 – –
Present×Female 283 14.93±0.14 – –
Includesonlythreebirthsperwoman.Adjustedformeanage(5yearsand10months)andmeanheight
(96.8 cm). Bold type indicates child’s weight was significantly different from the reference category,
controllingforageandheight
(inTable2),indicatingthatamaleinfant’sriskofdyingpermonthisdoubledifthe
biological father is not present in the household [OR=2.369, β=0.862±0.411,
p=0.036], while a female infant’s risk of dying is increased if the father is present
[OR=0.422, β=−0.862±0.411, p=0.036]. In Fig. 1, a Kaplan-Meier plot clearly
illustrates the relative and absolute higher monthly risk of dying for male infants
living in households without a father present. Fig. 2 illustrates the increased risk of
dying in infancy among females if their father is present.
Child Growth, 1–15 Years
The results of a multivariate generalized linear model to identify predictors of child
nutritional status up to age 15 (weight for height and age) are presented in Table 3.
Estimated marginal means for child weight, adjusted for mean age (6 years) and
Fig.3 Childweight(kg/m2)by
sexandfatherabsence
272 HumNat(2008)19:263–276
height(96.8cm),areshown.Sexisastrongpredictorofweight:femalesonaverage
are2kgheavierthanmales[β=3.49±1.19,p=0.004].Fatherabsenceonlybecomes
a statistical predictor of child growth when an interaction term with sex is added to
the model. A significant interaction term reveals that father absence has a positive
influence on female growth [β=3.44±1.21, p=0.005] but no discernable effect on
male nutritional status. This effect is illustrated in Fig. 3.
Discussion
Overall,despiteastrongculturalpreferenceforsonsamongtheArsiOromo,female
childrenareadvantagedovermalechildreninthestudy,contrastingsharplywiththe
notion that sons do better in male-biased, patrilineal societies (Pashos 2000). Arsi
Oromo females experienced a higher probability of survival during a critical period
in infancy, and accelerated growth during childhood, than males. Sex-specific
variation in infant mortality rates during periods of food scarcity, as was the case
during the study period, may relate to the difference in relative costs of early
development. Sons are both larger and more energetically expensive for the mother
toproducethanfemaleoffspring(Loosetal.2001;Marsaletal.1996).Importantly,
thecurrentstudyalsoidentifieswithin-populationvariationinpostnatalsurvivaland
health of sons and daughters, conforming to the Trivers and Willard (1973) and kin
selection (Alexander 1974; Hamilton 1964) models of evolutionary parental
investment theory. This will now be considered.
Parental Investment Protects Sons
Aboywholoseshisfatherconsidershimselflessthanothers(amaleinformant).
The effect of a father’s investment on infant survival among the Arsi Oromo varies
by sex. For sons, the extra resources and labor contributions of a father have a
positive impact on infant survival, while an absent father doubles a son’s risk of
dying in infancy. Conversely, an absent father improves a daughter’s chance of
survivingthismostdangerousperiodofearlychildhood.Otherstudiesexploringthe
impact of father absence on maternal child-care behaviors following marital
breakdown in the West indicate that single mothers pay more attention to daughters
thansons,butthisdeclineswhenthefatherispresent(Kowaleski-JonesandDunifon
2007; Mott 1994). Among the Arsi Oromo, sex-biased investment prior to weaning
is likely to be a product of a combination of factors, including prenatal investment
(low birth weight) and subsequent breast-feeding practices.
This finding appears to support an evolutionary Trivers-Willard-based model of
sex-biasedinvestmentduringearlychilddevelopment,withfatherabsence actingas
a proxy for parental condition/resource availability. Further, it is consistent with a
previous study undertaken by the author identifying a facultative adjustment in
prenatal investment (sex-ratio at birth), relating to maternal condition (Gibson and
Mace 2003). Such differential parental investment in sons and daughters within a
population could explain why many other studies, which do not consider sex