US higher-education institutions awarded a total of 55,006 research doctoral degrees in 2015, the highest number since the US National Science Foundation launched its annual Survey of Earned Doctorates in 1957. Three-quarters of those degrees were in science and engineering.

The number of recipients of science and engineering doctorates has more than doubled since 1975. And since 1995, the number of women receiving such doctorates has nearly doubled, with women earning almost half of the science and engineering PhDs awarded in 2015. The fastest-growing subfields for women were bioengineering and biomedical engineering, with an increase of more than 280% in PhDs awarded to women in those fields between 2005 and 2015.

Almost one-quarter of the science and engineering doctoral degrees awarded in 2015 were in the life sciences. Women earned slightly more than half of those.

In 2015, more PhD recipients of both genders reported intending to pursue postdoctoral research than were planning to pursue full-time permanent employment.

Female PhD recipients who had lined up postdoctoral positions reported being offered higher pay than did their male counterparts in a number of science and engineering fields, including agricultural sciences, health, physics and astronomy.

Of the doctoral recipients who were heading to non-postdoctoral positions, women were more likely to be working in academia (49%) than in industry (26%), whereas men's plans were more evenly split (42% academia, 39% industry). And more than half of men planning to seek such positions reported that research and development would be their primary activity, compared with 40% of women.

The vast majority of women who said they were heading to a non-postdoctoral position reported offers of lower salaries across all fields than did their male counterparts, with one exception: full-time jobs in physics and astronomy were offering women a median annual salary of US$97,750, compared with $95,000 for men.

For both men and women, the highest-paid science and engineering fields were mathematics and computer sciences, physics and astronomy, and engineering.

The report warns that the value of a US PhD may decline as universities in other nations step up their own doctoral programmes.