Or "unacknowledged parentage". "Unknown parentage" is a more common phrase but I think it changes the meaning too much.

This is tricky. I really don't find "intellectual property" convincing either in the title or in the text. In fact "propiedad" here means "ownership" rather than "property"; the point is that no one acknowledges being the "author" of the maras: that the maras are their "intellectual property".

I think a more workable approach might be to take our lead from "engendro", which basically means a foetus or a malformed, misshapen infant: "Criatura informe que nace sin la proporción debida". Of course it also means "Plan, designio u obra intelectual mal concebidos" (DRAE definitions), but here I think it's being used metaphorically in its basic sense: Salvadorian society has given birth to this monster but no one admits to being the parent.

When "propiedad intelectual" comes up in the text, lower down, you could perhaps translate it "intellectual parentage", which is an expression quite commonly used metaphorically in the opposite direction, as it were: to apply to authorship of a work or an idea.

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Note added at 1 hr (2013-09-03 12:51:34 GMT)
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By the way, though it seems rather non-PC, "monster" is applied to a teras or malformed foetus:

"5. monster, teras -- ((medicine) a grossly malformed and usually nonviable fetus)"
http://poets.notredame.ac.jp/cgi-bin/wn?cmd=wn&word=monster