One of my most excellent male colleagues currently has a research group that consists only of men -- all the research scientists, grad students, undergrad research assistants, and technical people are male, much to my colleague's dismay. It hasn't always been so, but in a male-dominated field, random fluctuations in a lab population can result in this situation. My colleague's fear is that he will be unable to attract women students or postdocs to his group because of it, and the all-male situation will self-perpetuate. He has been unable to recruit any women to his group for the past few years, the same length of time his group has been 100% male, although he has tried hard.
He has advised women students who have gone on to successful careers, but they went into industry rather than academia and so are not widely known in academia. His male graduates are all still in academia, so when academics think of my colleague and his former advisees, they think of these guys.
When recruiting prospective students and postdocs, my colleague highlights the accomplishments of his past students, not only because he is proud of them, but also in part to show that he has successfully advised a diverse group of students. He also doesn't mind being asked by prospective students about his record of advising women.
He is very aware that the dynamics in the lab are different when it's a group of guys, and he wants to change that, but it can be a challenge when he's competing for students and postdocs with some excellent women faculty at other institutions. If everyone felt like my colleague, just think how many great options there would be for women scientists -- and eventually these womenless research groups would no longer exist.
So, for anyone out there who is considering their options regarding potential advisors and research groups, if there are no women in the group currently, it's worth asking about the history of the advisor/group. If you don't feel comfortable asking the faculty member, you can probably get the information some other way -- from the director of admissions/graduate studies, other students, your undergrad advisor, or from some web-searching. You might find that the lack of women is an anomaly that the advisor wishes to fix, or you might find out that there is an insidious reason for it. Both are worth knowing so you can make the best decision.
7 years ago