🧠 Core Study: Hassett et al. (2008) – Monkey Toy Preferences

Welcome to one of the core studies in the Biological Approach! Hassett et al. investigated whether male and female rhesus monkeys display sex-differentiated toy preferences similar to those seen in human children, addressing the classic question: Are toy preferences biologically driven, or are they solely the result of human socialisation?

This study provides key evidence that certain preferences might be rooted in our biology (such as prenatal hormones and evolutionary history) rather than being purely learned from culture and marketing.

1. The Biological Approach Context

The Biological Approach assumes that our behaviour, cognitions, and emotions are primarily determined by physiological factors, such as the brain, hormones, genetics, and evolution.

Hassett et al. specifically addresses the Nature versus Nurture debate:

  • Nature: Toy preferences have biological origins (e.g. influenced by prenatal exposure to androgens and evolved behavioural biases).
  • Nurture: Toy preferences are shaped through human socialisation (e.g. parental reinforcement, gender stereotypes, advertising).

The core premise is that if non-human primates—who are not exposed to human gender socialisation—show the same sex-typed toy preferences as human boys and girls, then the behaviour is likely influenced by biological mechanisms (Nature).

Psychology Investigated:
  • Sex differences in play behaviour.
  • The role of hormones (specifically prenatal androgen exposure).
  • Biological origins of play behaviour versus socialisation.

2. Background and Aim

Background:

Human children consistently display sex differences in toy choices: boys tend to prefer wheeled/mechanical toys, while girls often choose plush/nurturing toys. However, in humans, biological factors are confounded with pervasive cultural socialisation.

Rhesus macaques (Macaca mulatta) provide an effective model to test biological influences because:

  • They share approximately 93% of their genetic sequence with humans.
  • They exhibit complex social and play behaviours without exposure to human cultural gender roles.

Aim:

To investigate whether rhesus monkeys show sex differences in toy preferences that parallel those observed in human children, testing the hypothesis that such preferences have a biological foundation.

3. Detailed Procedure (How the Study Was Done)

Hassett et al. conducted a field experiment with structured observation in the monkeys' outdoor troop enclosure.

A. Sample and Participants
  • Troop Size: 135 Rhesus macaque monkeys living in a social troop at the Yerkes National Primate Research Center Field Station.
  • Demographics: Included juvenile and adult males and females.
  • Active Sample: 82 monkeys interacted with the toys during trials; 34 monkeys (23 females, 11 males) engaged in ≥5 interactions and were included in the primary statistical preference analysis.
B. The Materials (Independent Variable)

The researchers used two categories of toys:

  1. Wheeled Toys (Masculine / Male-Typical): E.g. wagon, truck, car.
  2. Plush Toys (Feminine / Female-Typical): E.g. Winnie the Pooh, Scooby-Doo, Raggedy Ann.
C. The Procedure and Data Collection

1. Setting Up: Monkeys were tested in their large outdoor enclosure. In each trial, one wheeled toy and one plush toy were placed approximately 10 metres apart.

2. Observation Trials: Seven 25-minute trials were conducted and recorded using video cameras.

3. Data Collection (DV): Observers coded the frequency and duration of interactions with each toy type.

4. Measuring Interaction: Interaction was operationalised via a behavioural checklist, including holding, touching, sniffing, dragging, or manipulating the toy while within arm's length.

5. Social Rank: The dominance hierarchy/social rank of the monkeys was also recorded to assess whether troop rank confounded toy access.

4. Key Results and Conclusions

A. Quantitative Findings
  • Male Monkeys: Showed a strong, statistically significant preference for wheeled toys over plush toys in both interaction frequency and duration.
  • Female Monkeys: Showed no significant preference between plush and wheeled toys; they interacted with both toy categories and displayed greater variability in choice.
  • Age & Rank: Preferences were consistent across juveniles and adults. Social rank did not dictate the direction of toy preferences.
B. Conclusion

Sex differences in rhesus macaque toy preferences parallel those observed in human children (where boys show strong preference for wheeled toys, while girls show more flexible preferences). Because monkeys are free from human social pressures and gender role expectations, these findings strongly suggest that sex-typed toy preferences are driven in part by biological mechanisms (such as prenatal androgens and evolved perceptual/activity biases).

5. Evaluation: Strengths and Weaknesses

âś… Strengths of Hassett et al.
  • Controlled Observation in a Naturalistic Setting: Conducting trials in the outdoor enclosure allowed monkeys to interact naturally within their social group while standardising trial duration (25 minutes) and toy placement (10m apart).
  • High Reliability: Objective operational definitions for interaction and video recordings allowed for inter-rater reliability checks.
  • Controlling for Socialisation: Using non-human primates effectively ruled out human cultural pressures, advertising, and parental gender stereotyping as explanations for the male preference for wheeled toys.
❌ Weaknesses of Hassett et al.
  • Unequal Engagement: Out of 135 troop members, only 34 completed ≥5 interactions for the main analysis, and more females (23) participated than males (11).
  • Generalisability to Humans: Although rhesus macaques share high genetic similarity with humans, human cognitive development and cultural environments are far more complex.
  • Troop Dynamics: Dominance interactions within the outdoor enclosure could have influenced which subordinate monkeys were able to freely access the toys.

6. Issues and Debates

A. Nature Versus Nurture

This study provides evidence for the Nature perspective by showing that male preference for wheeled/mechanical toys exists in a species without human cultural conditioning, pointing towards prenatal hormonal influences. However, it does not imply nurture is irrelevant in humans; rather, biology may provide predispositions that culture and socialisation subsequently reinforce.

B. The Use of Animals in Psychological Research
  • Minimising Harm: The study was non-invasive, presenting non-harmful toys to animals in their familiar troop environment.
  • Replacement: Non-human primates were necessary to separate biological predispositions from the unavoidable socialisation present in human children.
  • Housing & Numbers: The animals were maintained in a large, enriched outdoor field station enclosure with their natural social group.
C. Application to Everyday Life

Understanding that male preferences for active/mechanical toys have biological roots can inform developmental psychology and education, helping parents and teachers recognise natural variations in play while still encouraging diverse, inclusive activities for all children.