The number of Americans taking antidepressants doubled to 10.1 percent of the population in 2005 compared with 1996, increasing across income and age groups, a study found. An estimated 27 million Americans ages 6 and older were taking the drugs by 2005, while their use of psychotherapy declined, according to Columbia University researchers. The findings highlight the need for doctors who are not psychiatrists and prescribe the medicines to be trained to diagnose and manage depression so patients get the most effective treatment, said the study’s lead author, Mark Olfson, a professor of clinical psychiatry at Columbia University and New York State Psychiatric Institute.