Economic model- minority sex sets rules of engagement for hetero sex-women who are minority on campus more likely to be virgins, even if they had boyfriends- it was as if they said “I’d rather not have sex yet & if you don’t like it there are plenty of other guys to choose from” 
Fewer men mean women compete for men with easier sexual access- “The general shortage of men on today’s campuses does seem to make it easier for men to demand and receive “low-cost” sex”
from Levay and Baldwin Human Sexuality 4th edition
from Levay and Baldwin Human Sexuality 4th edition
A “patriarchy model” would predict that if men were the majority on campus they’d create pressure for casual sex through coercion or intimidation- but we see the opposite pattern. When men are the majority women have less casual sex.
"These statistical significant associations [between sex ratio, attitudes and behavior] control for a variety of campus characteristics like region of the country, Greek life and whether it's a Christian College or not" books.google.co.uk/books?hl=en&lr…
