Hassett et al. (Monkey Toy Preference)

To understand why this study matters, you need the biology of sex differences — the part textbooks often state in one line.

Hormones come in two 'flavours' of effect.

• Organisational effects — hormones (especially androgens such as testosterone) acting during early development (in the womb) that permanently shape the structure of the brain and body. These set up later sex differences.
• Activational effects — hormones acting later (e.g. at puberty) that temporarily switch on behaviours in an already-organised brain.

The leading biological explanation for boys preferring 'masculine' toys is organisational: higher prenatal androgen exposure in males masculinises the developing brain, biasing later interests towards objects that move (wheels) over nurturing/soft objects.

Converging human evidence — CAH. Girls with congenital adrenal hyperplasia (CAH) are exposed to unusually high levels of androgens before birth. Studies (e.g. Berenbaum & Hines, 1992 — the children's data Hassett compares to) find these girls show more male-typical toy preferences. This is powerful because it links hormones, not socialisation, to toy choice in humans too.

Why study monkeys, then? Even with CAH evidence, human toy preference is tangled up with socialisation — children are praised/steered towards 'gender-appropriate' toys. Monkeys are not socialised about toys at all, so they let researchers test the biological idea with the social confound removed.

An evolutionary angle (use cautiously). Some argue males evolved a stronger interest in propulsive movement and objects (useful for hunting/tool use) and females in nurturing, which could bias toy interest. This is plausible but speculative — flag it as a possible explanation, not a proven one.

Psychology being investigated: sex differences, socialisation, play and the role of hormones.

Background. In humans, boys tend to prefer 'masculine' toys (cars, trucks) and girls 'feminine' toys (dolls, soft toys). But is this caused by biology (e.g. prenatal androgens) or by socialisation (children encouraged/rewarded for choosing 'gender-appropriate' toys)? With children the two are almost impossible to separate.

The clever idea. Rhesus monkeys are not socialised about toys the way children are — they have no concept of which toys are 'for boys' or 'for girls'. So if male and female monkeys still show different toy preferences, this points to a biological cause rather than learning.

Aim. To investigate whether the sex differences in toy preference seen in children also occur in rhesus monkeys (whose behaviour is assumed to be more biologically controlled), and so to test whether such preferences have a biological basis.

Sample: 135 rhesus monkeys (61 male, 74 female) living in a large, established social group at the Yerkes National Primate Research Center. No human participants were tested directly (children's data came from earlier published studies).

Toys (two categories):

• Wheeled ('masculine') toys — e.g. a wagon, a truck, a car, a construction vehicle, a shopping cart.
• Plush ('feminine') toys — e.g. soft cuddly toys such as Winnie the Pooh, a koala, an armadillo, a turtle.

Procedure:

• Toys were placed in the monkeys' outdoor enclosure (their normal environment — a fairly natural setting).
• Researchers observed the monkeys and coded their behaviour using a behavioural checklist with clearly defined categories (e.g. carrying, holding, manipulating, sitting on, mouthing the toy).
• The frequency and duration of interactions with each toy type were recorded across many observation sessions.
• The data were then compared with existing data on human children's toy preferences (to test the parallel).

Results:

• Male monkeys interacted significantly more with wheeled toys than with plush toys.
• Female monkeys interacted with both types of toy more evenly — they showed no significant overall preference (their pattern was less pronounced than the males').
• These patterns paralleled the toy preferences reported in human boys and girls.

Conclusion:

• Because monkeys are not socialised about toys, the sex difference must have a biological basis — most plausibly the organisational effects of prenatal androgens on the developing brain, and possibly evolved differences in interest (movement vs nurturing).
• Sex differences in toy preference are therefore not solely the product of socialisation; biology plays a real part — supporting the biological approach and the nature side of the debate, while still allowing biology to interact with social factors in humans.

Strengths

• Controls the socialisation confound — monkeys cannot be taught which toys are 'gender-appropriate', so a clear biological interpretation is possible (impossible with children).
• High ecological validity — observed in a natural social group/enclosure, so behaviour is more natural than in a lab.
• Objective behavioural coding with defined categories supports inter-observer reliability, and the finding converges with human CAH evidence.

Weaknesses

• Generalisability to humans — monkeys are not humans; human toy preference may still be heavily shaped by culture and learning, so extrapolation is uncertain.
• The toys are human objects with human labels — calling a wheeled toy 'masculine' imposes a human judgement that may be meaningless to a monkey (the monkey may like the wheels' movement, not its 'gender').
• Cause is inferred, not directly measured — hormones were not manipulated or measured in this study, so the hormonal explanation is an interpretation drawn from converging evidence.

Issues & debates

• Use of animals in research (KC2): discuss generalisability AND ethics (cost–benefit: minimal harm vs scientific value).
• Nature vs nurture (KC1): strong support for nature (biology), because socialisation is removed.
• Use of children in research: the comparison data come from children — consider why human data are harder to interpret.
• Application to everyday life: helps explain the origins of gendered behaviour and informs debates about gender, parenting and toy marketing.