Washington: In a disturbing trend, an internal Hubble Space Telescope (HST) study has found that in each of the past 11 observation proposal cycles, applications led by male principal investigators had a higher success rate than those led by women.
For an astronomer, winning precious observation time on the Hubble is a big deal - more than three-quarters of proposals are rejected.
It turns out, however, that this honour is a bit easier for men to achieve than women, the scientific journal Nature reported.
"I made a lot of efforts to have women on the review committees and during the review, I spent time listening to the deliberations of each panel. I never heard anything that struck me as discrimination so it is clear the bias is very subtle, and that both men and women are biased," commented Yale University astronomer Meg Urry who formerly led the Hubble proposal review committee for several years.
The Space Telescope Science Institute (STScI) in Baltimore, Maryland, runs the HST programme and began the study about two years ago.
After manually reviewing all proposals and categorising them by gender, researchers found that men's applications fared better than women's in every cycle they examined.
The effect is small - it translates to about four or five fewer proposals from women being selected each cycle than one might expect based on how many were submitted.
"You can kind of explain it away as just sampling statistics in any given cycle, but it happens every year," added Neill Reid, an STScI astronomer who oversees time allocation for Hubble.
The STScI has already implemented some changes to try to level the playing field for men and women.
The scientists who oversee proposal evaluation now tell reviewers before each cycle that this systematic effect exists, and that they believe unconscious bias might contribute to it.
"Sometimes people talk about the proposer rather than the proposal. We ask them to focus on the science," Reid added.
One positive development, the STScI team found, is that more and more women are applying for Hubble time.
The results are set to be published in an upcoming issue of Publications of the Astronomical Society of the Pacific.