The SOI measures restriction of sexual and pair-bonding behaviors. It has been validated on student populations around the world [77], [78]. Those who score lower on the SOI generally engage in sex later in relationships that are ‘characterized by reliably greater expressed love, dependency, commitment and investment’ [77, p. 876]. The SOI was slightly altered to differentiate heterosexual and homosexual activity and sexual identity, but because the sample contained only one subject self-identified as bisexual and four self-identified as homosexual, they were not distinguished from those self-identified as heterosexual in the further analyses.
