[Illustration: E.M. Lilien bookplate featuring satyr and woman]
This belief has its roots in a rabbinic tradition that believes demons (sheidim, creatures more akin to the Islam djinn than the earth-trembling terrors of Christian imagination) are unable to procreate without human “seed.” Thus Judaism has a robust tradition of succubae, seductive female demons who are the cause of male erotic dreams and nocturnal emissions. Adam was the first progenitor of demons:
When Adam, doing penance for his sin, separated from Eve for 130 years, he, by impure desire, caused the earth to be filled with shedim, lilin, and evil spirits (Gen. R. 20; Er. 18b).
Since, like humans, sheidim are subject to death (Chagigah 16a), these “semen demons,” such as Lilith, Naamah, and Igrat, periodically re-populate the demonic realm through these sexual-spiritual assaults.
The flip side of this coin is a parallel tradition that mortal women are occasionally impregnated by incubae:
Rabbi Hiyya Said: “sons of divinity” (Gen. 6:2-4) were the sons of Cain. For when Samael mounted Eve (Babylonian Talmud, Shabbat 146a), he injected [semen of] filth into her, and she conceived and bore Cain. And his aspect was unlike that of the other humans and all those who came from his side [of the human family tree] were called “sons of divinity” (Zohar I:37a;also see I:54a).
According to this version of the nefilim tradition, Cain was descended from an angel (Samael is called the "Prince of Heaven" in Pirkei de Rabbi Eliezer 13) but at the same time a “bad seed,” as were his descendants. The female descendants of Eve similarly can find themselves periodic victims of a kind of “Rosemary’s Baby Syndrome,” usually unknowingly. The changelings that result from such "spirit rapes" move among us largely undetected, until their evil nature is revealed through gross crimes or other evil enterprises.
Given the spiritual source of their malevolence, it was sometimes thought necessary to combat them by spiritual means alongside the usual police and judicial methods. Thus we see in some Hebrew amulets of protection that the person seeking angelic protection against evil spirits will identify him- or herself as “So-and-So, son/daughter of So-and-So, from among the children of Adam and Eve…” (Sefer ha-Razim). The implication being the amulet is directed against beings not from among the children of both primoridal ancestors.
This belief in demi-demon progeny persisted from Talmudic times right up to the start of the modern era, no doubt because this legend offers a ready explanation for why certain people are “bad to the bone,” much in the way we still today declare heinous serial killers and other violent criminal “monsters” (and therefore somehow not fully human).
To learn more, look up the Encyclopedia of Jewish Myth, Magic, and Mysticism available at Amazon. http://www.amazon.com/Encyclopedia-Jewish-Myth-Magic-Mysticism/dp/0738709050/sr=1-1/qid=1159997117/ref=sr_1_1/002-7116669-7231211?ie=UTF8&s=books