[greggarious]

You could select 57 random men, but the main issue is that smaller sample == higher variability.

Ever seen a political poll where one candidate has 51%, one has 49%, but the margin is ±3%?

A similar problem is probably present in many of the studies. Let's say you have 57 women. But not all of them experience motion sickness on the way up, maybe only a third. 20 people is a pretty small number, in statistical terms.

And the more uncommon the malady, the worse the problem gets.

Luckily, the solution is simple: more gender balanced space crews will generate more data.