Madsen (2007)
Kinship and altruism: A cross-cultural experimental study
Hamilton’s rule – individuals impose higher physical cost upon themselves when acting towards more closely
related individuals.
Altruistic behaviour originally proved difficult to explain in evolutionary terms: in a simple Darwinian world,
any gene promoting altruism should be rapidly driven to extinction. A solution to this dilemma was provided
by Hamilton (1964 – popularised by Dawkins, 1976) – who argued that evolution occurs as the result of copies
of genes being spread, and that replication of genes into future generations can occur through personal
reproduction, or the reproduction of those carrying the same genes.
Extension: Hamilton’s core finding was that a gene for altruism can evolve in a population:
rB > C
 B – is the benefit to the recipient
 C – is the cost to the altruist (measured as number of offspring gained or lost)
 r – The coefficient of relationship (i.e. that probability that they share the same genes)
Thus, more closely related individuals should be favoured over less closely related individuals (as they share
more of the same genes).
Three Experiments by Madsen (2007)
All three experiments followed the same repeated measures design. Participants were asked to impose a
cost upon themselves (in the form of pain from exercise), in return for a proportionate reward given to an
individual or organisation.
The exercise (shown right) was a wall squat. Participants were asked to hold
this position for as long as possible. The position becomes increasingly
painful with time and the pain incurred does not bear a linear relationship:
after approximately 100 seconds, pain increases massively.
The duration for which the participant held the position was recorded from
the moment the position was adopted, until the participant stood up or sat
down. The length of time the participants maintained the position on a given
trail was transformed into a material benefit for a pre-defined recipient.
The biological relatedness of the recipient was varied between four
categories:
 The participant themselves (r = 1)
 A sibling or parent (r = .5)
 A grandparent, aunt, uncle, niece or nephew (r = .25)
 A cousin (r = .125)
Participants were told that the proceeds would be sent to a nominated individuals (from the above 4
categories) chosen at random by the experimenter from among those listed by the participants).
At the end of each trial, the duration was determined and the fee calculated. Where the participants was the
beneficiary, this was paid directly to them; otherwise a cheque for the appropriate amount of sent.
Human Relationships
02 Kin Selection Theory
Prediction: If participants follow Hamilton’s rule, investment (time for which the position was held) should
increase with the recipient’s relatedness to the participant.
Experiment 1
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Participants recruited from student population at the University of Oxford.
Participants attended on successive days and carried out the experiment for one of the nominated
categories; £0.40 for each 20 seconds that the posture was held.
Following the trial participants rated how much the recipient would value the receipt of money.
Results
Participants imposed a higher cost when the
recipient was themselves (r=1) rather than r
= .50.
Likewise, participants imposed a higher cost
when the recipient was r = .50 rather than r
= .25.
No difference was found between r = .25
and r = .125.
There was a significant and reliable
relationship between the effort invested in
the task and the relatedness of the beneficiary, although there was no significant difference between the
last two related categories.
Experiment 2
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Participants were 20 male and 20 female students from the University of London.
The procedure was the same as above with additional categories: a same-sex best friends and a
national UK children’s charity.
Here participants were rewarded £0.70 for each 20 seconds.
Results
No reliable difference was found between r
= 1 and r = 0.5.
A reliable difference was found between r =
0.5 and r = .125.
No reliable difference was found between r
= 0.25 and r = .125.
Participants were no more willing to invest
in their best-friend that r = .125, but were
more willing to invest in their best-friend
than charity.
Human Relationships
02 Kin Selection Theory
Friends received an average of 15.4% more than other unrelated recipient (charity).
Experiment 3
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A replication of experiments 1 and 2 in two contrasting South African Zulu populations (Emmaus and
Hluhuwe), using only male participants, plus a control London group.
Participants attended on one day, and carried out five conditions, with a minimum of 15 minutes rest
between trials.
Participants were paid £1.50 per 20 seconds. The payments to Zulu participants consisted of food
hampers containing six items. For each 20 seconds they received an additional hamper. The charity
in this case was a local school, which received a cheque.
All Groups: All participants were more
willing to invest in themselves (r = 1) than r
= 0.50 and more willing to invest in r = 0.50
than r = 0.25.
There were no reliable differences between
r = 0.25 and r =.125 and r = .125 and charity.
Emmaus: There was a significant difference
between r = 1 and r = 0.50 but not between
r = 0.50 and r = 0.25.
Furthermore participants incurred a higher
cost for r = 0.25 vs r = 0.125 and similarly for
r = 0.125 vs. charity.
Hluhluwe: There was a significant different between r = 1 and r = 0.50 but no different between r = 0.50 vs. r
= 0.25 and r = 0.25 and r = 0.125. However participants were more willing to invest in their cousins, r = .125
than charity.
The results of experiment three confirm that the effect of kinship is trans-cultural. Furthermore, the results
provide experimental evidence in support of the claim that humans are more willing to incur costs for the
direct benefit of others, as a direct function of relatedness. These results demonstrate that humans behave
in such a way as to maximize inclusive fitness: they are more willing to benefit closer relatives than more
distantly related individuals.
Task/Question(s)
Could you summarise the aim, method, results and conclusion from the above? Would you need to? What is
important within the summaries given, and what is irrelevant to answering a question using the study?
In experiment 2 it was noted that participants were more willing to invest in their best friend in comparison
to charity. Comment on this finding in light of kin selection theory and any other theories of altruism.
Human Relationships
02 Kin Selection Theory