TOEFL Speaking Task 3

The concept of 'behavioral sink' refers to a situation where overcrowding leads to abnormal and often destructive behaviors in a population. This term was first coined by ethologist John B. Calhoun in his experiments with rats. When a population exceeds its optimal density, social structures break down, and individuals exhibit heightened aggression, withdrawal, or other maladaptive behaviors. The behavioral sink is not merely about physical crowding but about the psychological stress that results from too much social interaction without adequate space or resources, ultimately threatening the group's survival.

Professor: So, let me give you an example from a classic study. Calhoun built a 'rat utopia'—a large enclosure with plenty of food, water, and nesting areas, but no predators. He started with a small number of rats, and their population grew rapidly. However, as the density increased, the rats began to show strange behaviors. In the center of the enclosure, where the rats gathered most, males became extremely aggressive—they attacked each other and even females and young. Some males became 'withdrawn'—they stopped interacting and only came out to eat when others were asleep. Females neglected their pups, and some even abandoned them. There was also a group of 'probers'—rats that were overly active but unproductive. The social order collapsed, and eventually, the population stopped growing and began to decline, even though food and space were still available. This shows how overcrowding, not resource shortage, led to a behavioral sink.