According to the NSF's numbers, about 2/3 of doctorates awarded by U.S. universities in science & engineering fields go to U.S. nationals (citizens and permanent residents). The ratio has been fairly constant over the past 20 years, fluctuating within the 60-70% range. Not sure about prior to that.
In absolute numbers, there was a decline in U.S. nationals earning science & engineering doctorates from 1995 through 2002, but the trend is upwards again since 2002.
In absolute numbers, there was a decline in U.S. nationals earning science & engineering doctorates from 1995 through 2002, but the trend is upwards again since 2002.
Chart: http://www.nsf.gov/statistics/sed/2013/digest/theme1.cfm#cit...