A survey of aggressive behaviour among a population of persons with intellectual disability in Queensland.

A team mailed surveys to every agency that served adults with intellectual disability in Queensland, Australia.

They asked staff to record how many clients had shown any aggressive behaviour in the past year.

In total, 2,412 adults were counted across institutions, group homes and private homes.

Aggression showed up in 11 out of every 100 people.

The setting mattered. Big institutions had the highest rate: 35 in 100.

Group homes sat in the middle at 17 in 100. People living in their own homes had the lowest rate: 3 in 100.

S-Johnson et al. (2009) followed adults in the UK for two years and saw the same 10% starting point. They added good news: about 27% of those aggressive clients stopped showing aggression within the study period.

Hattier et al. (2011) ran a near-copy survey with 4,000 clients and found the same 10–11% rate, proving the number is solid across countries and time.

Cramm et al. (2009) dug deeper inside one facility and found most outbursts were triggered by social demands or tough tasks, giving staff a clear place to intervene.

Use the 11% figure when you write behaviour-plan summaries or justify funding.

Point to the 35% versus 3% gap when you advocate for community placement over large centres.

Pair the number with later remission data to show families that improvement is possible, then teach staff to ease task and social demands to lower the risk further.