more, regarding "how common?"

The brain gives itself a pat on the back for quickly detecting a
pattern among the faces. "It's good to make order out of chaos.
The brain rewards itself for finding something meaningful".
-- http://www.bu.edu/phpbin/news-cms/news/?dept=1127&id=41272

See also http://en.wikipedia.org/wiki/Koinophilia (the theories here
seem sort of heavily Darwinian and a little sketchy at that, but
interesting nonetheless). I also like the word, which is reminiscent
of the term 'koinomics' that my dad coined to talk about "the stuff
I study".

Anyway, this business about "how common" or "how average"
is interesting with respect to things like the job search. Skills
at Python and JavaScript are definitely better for most jobs than
skills at Lisp! Nevertheless, for the Lisp jobs that are available,
pay may be higher than average

(http://lispnews.blogspot.com/2008/08/lisp-job-market.html).