Sociosexuality as a concept was introduced by Alfred Kinsey to describe differences in individuals’ tendency to engage in uncommitted sexual relationships.

Simpson and Gangestad constructed the Sociosexual Orientation Inventory (SOI) in 1991. The SOI was constructed as a one dimensional bipolar measure that consisted of 7 items regarding past sexual behaviors, expected number of future sexual partners, frequency of sexual fantasies, and attitudes toward casual sex.


Sociosexuality was assumed to be a one dimensional construct, and the SOI score was calculated by aggregating all items using a formula proposed by the scale’s authors (Simpson & Gangestad, 1991). At one end of the continuum are highly restrictive individuals who require greater closeness and emotional bonding with a partner prior to engaging in sexual activities. At the other end of the continuum are highly unrestrictive individuals who require less emotional bonding with a partner before engaging in sexual intercourse. They feel comfortable engaging in one-night stands and having short-term relationships without emotional involvement (Gangestad & Simpson, 1990; Simpson & Gangestad, 1991).

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Sources:
https://en.wikipedia.org/wiki/Sociosexual_Orientation_Inventory
http://evp.sagepub.com/content/13/3/1474704915604541.full.pdf