Hi.
"Others contend that for a factor to be considered protective,
it must operate in the presence of an existing risk factor to reduce the negative impact of the risk factor. "
http://focus.psychiatryonline.org/article.aspx?Volume=2&page=558&journalID=21 Or, in other words:
"Others contend that for a factor to be considered protective, in the presence of an existing risk factor,
it must operate to reduce the negative impact of the risk factor."
Does 'oprate to reduce' mean 'able to reduce' here?