A study by Eric W. Hickey (2010) of 64 female serial killers in the United States indicated that sexual activity was one of several motives in 10% of the cases, enjoyment in 11% and control in 14% and that 51% of all U.S. female serial killers murdered at least one woman and 31% murdered at least one child. In other cases, women have been involved as an accomplice with a male serial killer as a part of a serial killing team. A 2015 study published in "The Journal of Forensic Psychiatry & Psychology" found that the most common motive for female serial killers was for financial gain and almost 40% of them had experienced some sort of mental illness.