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Last updated 31.10.11.
Introduction
From an
evolutionary perspective, offspring are the vehicle for parents’ genes, so
selection mechanisms should favour parental care strategies designed to ensure
the survival and reproduction of offspring. Hrdy has introduced us to the idea
of ‘proximity-promoting’ behaviours in babies that ensure they are cared for,
and attachment theorists such as Bowlby and Ainsworth argue that the quality of
care a child receives in its earliest years is crucial for their subsequent
ability to develop and sustain social relationships in later life. Combined,
these theoretical perspectives have been used to support a worldview that
emphasises the centrality of the primary carer (usually the mother) in a
child’s formative years, and by implication, assumes that the mother-child
relationship will (should) be harmonious.
Certainly,
our contemporary view of family dynamics in the west assumes that conflict
within the family is indicative of a ‘dysfunctional’ family unit. However, this
week’s session deals with Trivers’ assertion that this interpretation is
mistaken, and the reverse is true: that conflict between mammalian parents and
offspring is inevitable. Trivers argues that the conflicting genetic and
evolutionary imperatives for:
·
adults to bear several offspring in order to maximise the potential for
reproducing their genes
·
each offspring to secure its survival enlisting the care of its parent
mean that
each individual offspring is always going to demand more parental investment
than the parent is willing or able to give.
Moreover, Trivers
(who is an evolutionary psychologist) states that this conflict begins very
early in the child’s life, citing weaning as the initial classic conflict between
parent and child: the parent desires another baby and needs to wean the existing baby from the
breast in order to conceive, but the existing child does not want breastfeeding
to cease and uses the psychological tactics available to him/her to prevent this
happening.
Trivers
introduces two concepts into debates about childcare:
1.
parental investment; and
2.
parent-offspring conflict
Parental Investment and Sexual Selection (1972)
Parental
investment is anything that a parent does to promote the survival of an
offspring, which also detracts from the parent’s ability to invest in the other
offspring. Trivers defines parental investment as:
‘any investment by the parent in an individual
offspring that increases the offspring’s chances of surviving […] at the cost
of the parent’s ability to invest in other offspring.’ (Trivers, 1974 cited in
Barrett, Dunbar and Lycett, 2002; p. 34)
He is
assuming that the majority of parental investment will be undertaken by
mothers, and employs evolutionary psychology to offer 3 hypotheses why this
might be the case.
1.
Paternity Uncertainty reduces the male’s direct
investment in children
2.
Abandonability of the female reduces the male’s indirect
investment in children
3.
Limited Mating Opportunities for males who invest exclusively in
one family
The outcome
of this male ambivalence is that females have to be more
selective
about who they mate with than do males: if they choose poorly they are likely
to be left alone literally holding the baby.
Parent-Offspring Conflict (Trivers, 1974)
Trivers
challenges the conventional psychoanalytic wisdom regarding parent-child
conflict that was offered by Freud in his notion of the Oedipus Complex
(1900/1953). Freud hypothesised that between ages 2 and 5 boys develop a sexual
attraction toward their mothers, which puts them in conflict with their father
who has actual sexual access to the mother, leading to the son’s unconscious
desire to kill his father. This hypothesis assumes same-sex conflict within a
family.
Triver’s
theory of parent-offspring conflict does not place great significance upon the
child’s gender, and suggests instead that conflicts are driven by disagreements over the allocation of the
parents’ investments. Trivers argues that Freud’ Oedipal complex hypothesis is
mistaken on two counts:
·
There may be conflict between
fathers and sons, but this would be over parental investment, not sexual access
·
Any sexual conflict between fathers and sons is likely to be concerned
with sexual access to other women, not the mother
Trivers
(1974) offers an alternative explanation for parent-child conflict, which has
no gender bias but challenges the assumption that:
‘Parents are
classically assumed to allocate investment in their young in such a way as to
maximise the number surviving, while offspring are implicitly assumed to be
passive vessels into which parents put the appropriate care.’ (Trivers, 1974
cited in Hrdy, 1999: pp. 426-7)
Trivers
suggests that if the offspring are perceived as actors in this interaction then
they can be assumed to experience conflict between themselves and their
parents, and between themselves and their siblings, as each offspring demands
more parental investment than their parents are willing to give.
The issue
here is about the subconscious decisions parents make about the allocation of
resources between their children, or, between their existing child and other
children they are planning to have. Trivers is not arguing that these decisions
are made frivolously (this is not an argument about favouritism) but are driven
by survival imperatives.
Because any
child shares only half of its genes with each of its parents, siblings, parents
and offspring – so while these individuals are usually perceived as being
‘related’ in significant ways they can also be perceived as differing genetically to a significant
degree. The difference between siblings
is crucial to Trivers’ argument, because the conflict between parents and
offspring that he describes is generated by competition between siblings.
Genetically,
parents have an equal investment in each of their children - even though the
children may differ greatly in their characteristics (and therefore their
similarity to each parent), and more importantly, in their potential to
survive. Parents’ perspective on the ideal allocation of resources within the
family would be to share them out equally among their offspring. However, each
offspring wants the whole allocation for his or herself, and doesn’t share
their parents’ investment in their siblings’ survival because they are not
sufficiently genetically similar to them. Investment by a parent in one
offspring detracts from what is available to her other offspring, and therefore
parents and offspring are likely to disagree over the allocation: each child
desires a larger portion of the parents’ resources than the parents want or are
able to give.
Trivers
proposes that parents are likely to continue to invest in an offspring up to
the point where the costs of this investment outweigh the benefits to the
offspring. At this point, it makes evolutionary sense for the parent to reduce
their investment in the current offspring and start investing in the next. This
leads to three specific hypotheses identified by Buss (1999) who has developed
Trivers’ work for a contemporary audience:
1.
Parents and children will get into
conflict about the time at which the child should be weaned, with parents
wanting this to happen earlier than the child wishes it
2.
Parents will encourage children to
value their siblings more than children are naturally inclined to value them
3.
Parents will tend to punish conflict
between siblings and reward co-operation
Consequently,
as Hrdy notes, children have to resort to emotional and psychological manipulations
in order register their disappointment at the type of parental investment they
are receiving and present their case for greater parental investment.
‘Tantrums are the most dramatic, outward, and visible
manifestation of these tactics. Though they appear to be spontaneous outbursts,
Trivers believed tantrums could be quite calculated. Infants engage in
manipulative exaggeration of emotional and physiological states.’ (Hrdy, 1999;
p. 429
When does the conflict begin?
Trivers’
argument is significant because it undermined the contemporary popular view
that relationships between parents and young children are essentially warm and
joyful, and that parent-offspring conflict occurs in humans occurs when the
child is older, and reaches its peak when the child is an adolescent. However,
David Haig, an Australian evolutionary psychologist suggests that Trivers’
argument doesn’t reach far enough by failing to recognise the extent of
conflict that occurs between mother and offspring even before birth.
Although it
is in both the mother’s and the child’s interests for the pregnancy to progress
successfully, their individual interests regarding the pregnancy don’t neatly
coincide. Haig goes so far as to suggest that far from being the harmonious
event that the manuals describe, pregnancy is a struggle for control between
the mother and the baby. Palmer and Palmer (2002) summarise the problem for the
mother as follows:
‘From the perspective of the mother’s genes,
investment in the current fetus must be weighed against investment in existing
children or in future children. If conditions are such that investment in the
current fetus will severely compromise the probability of survival of existing
children or offspring, then it is in the mother’s interest not to invest in the
fetus.’ (Palmer and Palmer 2002: p. 145)
However, the
foetus does not have to make the same kind of calculations: it simply has to
exert whatever physiological influences it can upon the mother to ensure its
survival. This is achieved through the work of the placenta, which implants
within the uterine lining in order to control the supply of nutrients to the foetus, and is viewed by obstetricians
as:
‘a ruthless parasitic organ existing solely for the
maintenance and protection of the fetus, perhaps too often to the disregard of
the maternal organism.’ (Page 1939, cited in Hrdy 1999; p. 433)
Four main
problems (or ‘inefficiencies to use Haig’s term) associated with pregnancy can
be seen to support Haig’s argument:
1.
Miscarriage
2.
Morning Sickness
3.
Gestational Diabetes
4.
Pre-eclampsia
Miscarriage
As many as
75% of all fertilised eggs may fail to implant or are spontaneously aborted in
early pregnancy (Nesse and Williams, 1994), often before the woman is even
aware she is pregnant, and usually because of some form of chromosonal
abnormality in the foetus.
After an egg has been released by the ovary, the corpus luteum (which is formed from the follicle of the released egg) produces declining levels of progesterone for the next two weeks. If there is no embryo (fertilised egg) in the uterus, this declining progesterone production results in menstruation. If an embryo is embedded in the uterus it will produce human chorionic gonadotrophin (hCG), which serves to keep progesterone levels high, and thereby prevent menstruation. Haig (1998) suggests that we perceive the embryo as an independent entity asserting its right to survive, and thus resisting the normal processes of the woman’s reproductive cycle in order to achieve this goal. He proposes that the ability to produce high levels of hCG is an indicator of the ‘health’ or ‘quality’ of the embryo, and that embryos which are not strong enough to counter the woman’s production of progesterone are not worth the mother’s investment because they suggest that the genetic material they carry is poor. Consequently they are shed at the usual time of menstruation: the woman wins this particular battle and thus retains the possibility for more successful reproduction of her genetic material at a later date.
Morning Sickness
A large
proportion of women (at least 75% according to Tierson, Olsen and Hook, 1986)
develop a heightened sensitivity to smell and a nauseous reaction to certain
foods in the first trimester of pregnancy. Our usual terminology for this
phenomenon is ‘morning sickness’, implying that something is amiss
physiologically with the mother. However, a large body of research evidence is accumulating
to suggest that the sickness is an adaptive behaviour designed to protect the
developing foetus from harmful toxins that be found in many of our ‘everyday’
foods and can be responsible for birth defects and miscarriages: apples,
bananas, potatoes, celery, black pepper to name just a few. Specific foods that
pregnant women develop aversions to are: coffee; vegetables; meat; eggs and
alcohol. All of these foods contain high levels of toxins, and the stage of
pregnancy when women are most highly sensitised to them is when the toxins
would have the most harmful effect upon on the foetus’s organ development. The
most significant piece of evidence to support this hypothesis of the valuable
role morning sickness plays in pregnancy is that women who do not experience
nausea in the first trimester are three times more likely to miscarry than
women who do have morning sickness, and also produced babies with higher birth
weights (Profet, 1992).
However, it
is not clear whether this process is embryo- or mother-driven. That is, whether
the mother’s body becomes sick to protect the embryo, or the embryo exerts
physiological pressure on the mother to protect itself. Once again, Haig
suggests we need to reconsider our assumption that morning sickness is evidence
of the woman’s body’s protective behaviour towards the foetus, and instead
perceive the foetus as an independent entity exerting its right to survival
upon the woman’s body, with little regard for the distress this might entail
for the woman.
Gestational Diabetes
Pregnant and
non-pregnant women’s blood sugar levels differ significantly. When she is not
pregnant, a healthy woman’s blood sugar levels will rise and fall relatively
quickly after eating carbohydrates.
Maternal blood sugar levels typically drop during early pregnancy and
then stabilise at this low level for most of the pregnancy. However, in the
advanced stages of pregnancy, a woman’s blood sugar level rises to higher
levels after eating carbohydrates, and stay elevated for longer. This is
because the placenta releases human placental lactogen (hPL) which increases
the woman’s resistance to the insulin produced by her pancreas to stabilise her
blood sugar levels. The purpose of stimulating this increased insulin
resistance in the mother is to ensure that the foetus can receive the sugar it
requires to achieve a healthy birth weight.
Usually, an
appropriate balance is achieved for both mother and foetus, but in a
significant minority of cases the insulin resistance stimulated by the hPL can
produce temporary diabetes in the woman, so that she has to carefully control
the amount of sugar she ingests in order to keep healthy and avoid the possibility
of developing insulin-resistant diabetes permanently. This is a chronic
incurable disease that leads to cardiovascular disease, kidney disease and
neuropathy. The foetus is not harmed by gestational diabetes, although its
increased weight can make birth difficult. What Haig appears to be suggesting
here is that the placenta works for the benefit of the foetus, even though this
may cause temporary (and even long-term) serious health problems for the
mother. This suggestion supports the argument for foetus-mother conflict before
birth, but has a fatal flaw: if the rise in insulin resistance promoted by the
foetus is very high, the baby will be very large and therefore at risk of harm
during delivery. Moreover, if the rise in insulin resistance promoted by the
foetus so great that it makes the woman seriously ill permanently, it is also
likely that the woman’s capacity for securing her baby’s survival will also be
compromised.
Pre-eclampsia
In the early
stages of pregnancy, a the woman experiences a drop in blood pressure as the
placental cells destroy her arteriolar muscles that are responsible for
adjusting the flow of blood to the foetus, and remodels the endometrial spiral
arteries so that the placenta gains control over its source of nutrition. It
achieves this by releasing a protein called neurokinin B (NKB) that causes
veins to restrict and the heart rate to increase, both of which raise blood
pressure, which has the effect of delivering more nutrition to the foetus. This
process usually results in a subsequent slight raise in blood pressure for the
mother, which she manages with ease. However, a more dramatic rise in blood
pressure is set in motion when the foetus is insufficiently nourished, either
because the mother is malnourished, is older, or because the placenta is poorly
implanted in the uterus.
Some
increase in blood pressure in pregnancy is harmless for the mother (and is also
linked with a lower rate of miscarriage Haig, 1993), and in these cases the
foetus appears to have the upper hand in the ‘conflict’ between woman and
foetus. However, very high blood pressure can lead to a condition in the mother
called pre-eclampsia, which if left untreated, can lead to eclampsia, an acute
physiological crisis that not only generates a range of life-threatening conditions
such as kidney failure, liver failure, heart attack and stroke in the mother,
but is also highly likely to lead to the death of the foetus.
Currently,
the only way to alleviate the symptoms of pre-eclampsia is by delivering the
baby as soon as possible. About a third of all premature births are caused by
pre-eclampsia: some of these will have happened naturally, others will have
been induced early as the obstetrician has calculated that the risk to mother
and foetus is too great to allow the pregnancy to continue, and others will
have been delivered by caesarean section because the mother and/or foetus is
too vulnerable to endure a natural delivery. In these cases, the foetus’
demands on the mother’s physiology appear to have backfired, as it has less
than the desired time in the womb and has also been poorly nourished during
that time. It is also highly likely that the damage cause to the woman by the
stresses placed upon her body in eclampsia and severe pre-eclampsia will
compromise her ability to invest in her child’s survival.
Delivery
The average
human pregnancy lasts 40 weeks, and it is easy to assume that the foetus has
this time period ‘pre-programmed’ in some way. However, Hrdy proposes that the
date of birth is much more provisional, stating:
‘The exact moment of birth can be viewed as a
negotiated settlement between embryonic demands and maternal assessments of the
environment that the will both inhabit.’ (Hrdy, 1999; p. 434)
A hormone
called Cortiotropin Releasing Hormone (CRH) appears to be responsible for
determining the duration of pregnancy. In a normal pregnancy, the placenta
releases CRH, which stimulates the pituitary of the foetus to secrete
adrenocortiotropic hormone (ACTH), which acts on the adrenal glands of the
foetus so that they release oestrogen. This raised level of oestrogen causes
the muscles of the uterus to synthesise connections and stimulates the mother
to secrete oxytocin, which stimulates contractions in the uterus and the
production of milk.
In
problematic pregnancies such as those described above, the placenta is likely
to stimulate this process prematurely. Haig (1993) argues that this ‘cutting of
losses’ occurs when the supply of nutrients to the foetus becomes so
unsatisfactory that the survival of the foetus becomes unlikely. The cost to
the foetus of remaining in an unsupportive environment becomes too great, and
premature entry into the world seems preferable, not least because the threats
to the mother’s health also threaten the foetus’ future survival. In
evolutionary terms, the premature arrival of an under-nourished child to a
mother who is strong enough to care for it is preferable to a slightly
stronger, but motherless child.
References
Barrett, L.,
Buss, D.
(1999) ‘The theory of parent-offspring conflict’ Evolutionary Psychology: the New Science of the Mind Allyn and
Bacon:
Haig, (1993)
‘Genetic Conflicts in Human Pregnancy’ The
Quarterly Review of Biology 68 pp. 495-532
Hrdy (2001)
‘The past, present and future of the human family’ The Tanner Lectures on Human Values delivered at the
Trivers, R.
(1974) ‘Parent-offspring Conflict’ American
Zoologist 14 pp. 249-264
Bibliography: Trivers, R. (1985) Social Evolution CAL: Cummins Publishing; your hand-out of chapter summaries comes from this.