When I first joined Carleton, just three out of thirty professors in the economics department were female. Then one retired, one moved to Montreal, and I was the only female professor around on a regular basis.
For the most part, I didn't mind. If I was the kind of person who objected to being in a male-dominated profession, I wouldn't have gone into economics in the first place. But the absence of women meant I had to socialize with male colleagues, or not socialize at all. I had to work out what kinds of interactions with my male colleagues were socially or otherwise acceptable. It wasn't obvious, at first, where to draw the line.
Anything - lunch, coffee, drinks - as long as it's done with a group of three or more?
Can a man and a woman have a conversation in an office with the door open?
A tête-à-tête in an office with the door closed?
Is it acceptable for a female colleague and a male colleague to have coffee alone together?
Lunch together on-campus?
Lunch together off-campus?
Dinner? Dinner out of town at a conference?
How does any of this change if the people involved are co-authors? Working together in a professional capacity, for example, as chair and graduate advisor, or dean and associate dean?
In practice, I draw the line between lunch on-campus (acceptable) and lunch off-campus (unacceptable), with exceptions for extreme circumstances. I'm not suggesting this as a universal rule. Carleton's particular geography makes going off-campus for lunch a special occasion, which is not true for other universities.
Yet if it's okay for two male colleagues to go out for lunch, drinks or dinner together, but it's not acceptable for a male and a female colleague to do the same, the minority sex will tend be isolated and marginalized.
There are things that both men and women can do to prevent this marginalization.
One possibility is to adopt a policy of strict symmetry - don't do anything with a male colleague that you wouldn't do with a female colleague, and vice versa.
Sheryl Sandberg advocates this solution in Leaning In, when she discusses men mentoring junior colleagues. It works for relationships with students. Here, like most profs, I have my own set of unwritten rules. No Facebook friendships with undergrads (easy to implement, except when someone who is already a Facebook friend decides to major in economics). Never (o.k., hardly ever) close the office door, and so on.
The strict symmetry approach doesn't work for academic colleagues - it would interfere too much with too many friendships. But it's still not a bad starting point. As of July 1, I'll be stepping into the Associate Dean position that Nick Rowe is currently filling. The other day I said to my soon-to-be-opposite number in engineering "we should have lunch together." I think he was taken aback. (He might not have been. It can be hard to tell with engineers.) But why? (Perhaps engineers don't do lunch?) Still, if it would be appropriate for him and Nick to have lunch together, why wouldn't it be appropriate for him to have lunch with me? At what point do concerns about propriety start interfering with collegiality and collaboration?
Group activities are another way of creating an environment where no one feels marginalized. If I had to give advice to a junior colleague who was feeling lonely, it would be to organize some group activities. It's easy to say, "the department should do this." The department is us. If individual people within the department don't step up, nothing will happen.
Anyone who hopes to thrive in a mostly-opposite-sex environment also has to develop some toughness. Something that I personally struggle with is walking into the grad student bar on my own. I dread that moment of standing there, feeling uncomfortable, looking for the economics gang. I can't shake years of conditioning: nice girls don't go into bars on their own. But nice girls don't usually become economic professors. So I just make myself walk through the door.
Worrying about the etiquette of lunching with opposite-sex colleagues seems so last century. But I don't think the issue has gone away. In some ways, it's harder for my junior colleagues than it was for me.
The scarcity of female colleagues forced me to reach out to male colleagues. Now that there are more women in the department, it's easier for us to stick together. But that can make the men in the department feel uncomfortable - the sight of three or four women talking together is enough to make people stop and wonder what diabolical plots are being hatched. (Seriously. People have been known to stop and comment.) And while some men are comfortable in mostly female settings, not all are.
The economics profession is also more diverse than it once was. This is a good thing, but it means that people have to make an effort to reach over cultural and language barriers, as well as gender barriers. I think the fact that I grew up in Canada, and have a northern European background, makes it easier for me to do things like buy a round of drinks in a bar, or invite an opposite-sex colleague out for coffee, than it would be for a person who grew up with more traditional gender roles.
As a person becomes professionally more established, too, new challenges arise. Is it okay to go out to for dinner with a co-author in another city? To go out drinking with a former supervisee? Nick Rowe, in his position as Associate Dean, will go out for drinks with the (male) Dean. But is it appropriate for a female Associate Dean and a male Dean to go out drinking?
On the other hand, if one's most serious professional concern is who to have lunch with, life is pretty good.