The use of antidepressants has increased dramatically over the past 25 years, says Michael Thomas of Idaho State University in Pocatello.
Around 80 per cent of each drug passes straight through the human body without being broken down, and so they are present in waste water.
In most communities, water purification systems cannot filter out these pharmaceuticals. "They just fly right through," says Thomas, which means they ultimately find their way into the water supply.
The concentration of these drugs in drinking water is very low – at most, they are present at levels several orders of magnitude lower than the prescription doses.
But since the drugs are specifically designed to act on the nervous system, Thomas hypothesised that even a small dose could affect a developing fetus.
...Although the researchers had expected the drugs might activate genes involved in all kinds of neurological disorders, only 324 genes associated with autism in humans appeared to be significantly altered.
Most of these genes are involved in early brain development and wiring.