When used positively, and particularly when used as a preference or description by someone else, it is intended to imply a male who puts the needs of others before his own, avoids confrontations, does favors, gives emotional support, tries to stay out of trouble, and generally acts nicely towards others.
In the context of a relationship, it may also refer to traits of honesty, loyalty, romanticism, courtesy, and respect.
Though this is the origin of the phrase, Durocher's remark was specific to the context of baseball, and indeed to the context of that set of players, rather than intended as generally applicable to male/female relationship dynamics or in any other context and his allegation of a cause-and-effect relationship between being nice and finishing last was at most merely implicit – it can also be interpreted as "Nice guys, but they will finish last", rather than "all nice guys finish last".
This was disputed by Richard Dawkins, who wrote the book The Selfish Gene.
A 2008 study at New Mexico State University in Las Cruces showed, that "nice guys" report to have significantly fewer sexual partners than "bad boys".
Barclay (2010) found, that when all other factors are held constant, guys who perform generous acts are rated as being more desirable for dates and long-term relationships than non-generous guys.
In two studies, Urbaniak and Kilmann found that women claimed to prefer "Nice Todd" over "Neutral Todd" and "Jerk Todd", relative to "Michael" even at differing levels of physical attractiveness.
They also found that for purely sexual relationships, "niceness appeared relatively less influential than physical attractiveness." After acknowledging that women's preference for "niceness" could be inflated by the social desirability bias, especially due to their use of verbal scripts, they conclude that "our overall results did not favor the nice guy stereotype." Mc Daniel (2005) constructed vignettes of dates with a stereotypical "nice guy" vs.