Relationships:
Evolutionary Explanations:
● Each living thing (organism) has a set of characteristics encoded by its genes
○ The organism inherits its genes from its parent(s)
○ Variations between organisms are caused by variations in the genotype
● Characteristics evolve because they have been passed from one generation to the next via
the genes
○ This requires the organism to:
■ Live to reproductive age
■ Mate successfully
■ Ensure the offspring’s survival
● Sexual selection explains why some characteristics might appear disadvantageous actually
confer an advantage in human reproductive behaviour because the characteristics are
attractive to potential males
○ Individuals possess features that make them attractive to the opposite sex or help
them compete with members of the same sex for access to mates
■ These festures mean that the possessor breeds successfully - traits
concerned with increasing reproductive success are naturally selected and
retained
■ Any trait that increases reproductive success of an individual will become
increasingly exaggerated over evolutionary time
● Natural selection → ability to survive in a particular environment (fitness)
○ These are separate processes - the characteristics that facilitate reproduction do no
necessarily increase survival chances
● Sexual dimorphism → the differences in appearance between males and females of the
same species, such as in colour, shape, and size
● Anisogamy: differences between male and female sex cells (gametes)
○ Since the female invests more in the production of an egg than the male does in the
sperm, she is going to be discriminating in her choice of mate
■ Both sexes are choosy because they both stand to lose if they invest
resources in substandard partners, but the consequence of making the
wrong choice of partner are much more serious for the female
○ The female egg cell is much more costly to produce than a male sperm cell
○ Females must provide nourishment for the offspring for 9 months, which means she
can only have a limited number of offspring
■ By contrast, a male can have a virtually unlimited number of offspring
■ Anisogamy is also important in partner preference because it gives rise to
two different mating strategies, which means there are two types of sexual
selection
● Intersexual selection → the preference of one sex for the member of the opposite sex who
has certain qualities
○ This is the preferred strategy of the female - quality over quantity
, ○ The female has a preference for a fit male which determines which features are
passed onto the offspring
■ This is known as the ‘runaway process’ → an adaptive feature will be
selected for when females choose a sexual mate
● This may result in a feature becoming exaggerated over many
generations
■ This leads to the ‘sexy sons hypothesis’ where a female mates with a male
who has desirable characteristics, and this ‘sexy’ trait is inherited by her son
● This increases the likelihood that successive generations of females
will mate with her offspring
○ Characteristics indicating good resources: economic/financial security, good job, well
educated, interpersonal skills, status, etc
○ Characteristics indicating good genes: good health, soft skin/hair, weight, facial
symmetry, but also traits that are disadvantaged (peacock’s tail)
○ Characteristics indicating good parents: good skills, caring, affectionate, strong,
protective, ability to provide/nurture, etc
○ Intersexual selection has psychological and behavioural consequences
■ For example, for males to acquire females and protext them from competing
males, they may benefit from behaving aggressively and perhaps even
thinking in a certain way
■ A behavioural consequence is a distinct preference for youth and a
sensitivity to the indicators of youth as well as fertility
● Fertility → probability of reproducing now
● Reproductive value → probability of reproducing in the future
○ Youthfulness would be an indicator of both reproductive value and fertility
■ This could be signalled by physical characteristics such as smooth skin, good
muscle tone, full lips, healthy hair, or by behaviour such as high energy levels
or a sprightly gait (Symons, 1979)
● Intrasexual selection → members of one sex compete with each other for ‘access’ to
members of the opposite sex - victors mate and pass on their genes
○ This is the preferred strategy of the male - quantity over quality
○ This strategy has given rise to dimorphism in humans - the obvious differences
between males and females
● Intrasexual selection - Buss (1989):
○ The aim of the International Mate Selection Project was to identify the
characteristics that individuals valued in potential mates worldwide
■ Used questionnaires developed in the USA about what people looked for in a
partner
■ Asked over 10,000 people from 37 different cultures in Europe, Africa, and
the Americas
● Participants were asked to rate each of 18 characteristics (physical
attraction, good financial prospects, etc) on a 4-point scale - 0 =
irrelevant, 3 = indispensable
○ Found that women:
■ Value earning potential more than men cross-culturally
, ■ Value ambition and industriousness more than men
■ Prefer older (higher status) males
○ Found that men:
■ Value physical attractiveness more than women
■ Value chastity more than women
■ Prefer younger women (typically 3 years younger than themself)
○ Found that both wanted intelligent (parenting skill) and kind (long-term)
○ - The study suffered from problems of translation in the questionnaires, which could
decrease validity of the results
○ - The samples for each country were not representative so it is impossible to
generalise the findings
○ - One of the findings was that males preferred females who were already well into
adulthood → contrary to evolutionary predictions
○ - Anderson (1986) showed that fertility may peak earlier in females than previously
thought, perhaps as early at the mid-twenties → perhaps men value fertility in a
mate more than reproductive value (potential for child-bearing years)
○ - Rural and less well-educated individuals were under-represented in this study, and
the sampling method varied widely across cultures
○ - In many cultures, arranged marriages are the norm → if these date back to our
ancestral environment, how could mate preferences have evolved in the first place
● Clark and Hatfield (1989):
○ Attractive male and female experimenters approached strangers on a college campus
and said, “Hi, I’ve been noticing you around campus and I find you very attractive.”
■ They then asked them one of these questions:
● Would you go on a date with me tonight?
● Would you go back to my apartment with me tonight?
● Would you have sex with me tonight?
○ They found that of the females approached, 50% agreed to go on a date with the
man, 6% to his apartment, and 0% to have sex
■ Out of the males approached, 50% agreed to a date, 69% were willing to go
back to her apartment, and 75% agreed to have sex
○ Men have evolved psychological mechanisms to ensure success in short-term mating
■ These include a desire for sexual variety, the tendency to let little time
elapse before sexual intercourse, and a willingness to consent to sex with
strangers
○ Buss and Schmitt (1993): men tend to lower their standards in teh context of
short-term mating opportunities, and show a marked decrease in attraction for their
‘casual partner’ after copulation
■ This is an evolved adaptation to ensure they don’t spend too long with one
woman, and so can move on to the next
● Partner preferences over the past century have undoubtedly been influenced by rapidly
changing social norms of sexual behaviour
○ These develop much faster than evolutionary timescales imply and have instead
come about due to cultural factors, such as availability of contraception
