Plans revealed this week to squeeze a further 1.1 million people into Sydney over the next 25 years will transform it into the nation’s least liveable city. Twenty years ago Sydney was less congested, slower, more friendly and had more green space. Unregulated population growth and timid planning are choking the city, a situation exacerbated by the refusal of the Labor Government – still hostage to the economic rationalist fear of public debt – to invest in a modern public transport system. A famous experiment in the 1960s found that when too many rats are forced to live in a cage of a given size they soon display abnormal behaviour including hyper-aggression, failure to nurture young normally, increased mortality, abnormal sexual patterns and infant cannibalism. In human populations, crowding causes physical diseases and psychological stress. When humans are forced to live in these “behavioural sinks”, defined less by geographical proximity than by excessive social interaction, they respond with aggression, attempts to isolate themselves and, when all else fails, drugs and alcohol.