According to this survey done by Zogby International. The numbers:
31% “Other” (the pollsters presume this is mostly those with “No religion”)
The sample size was small, only around 400. And it seems really strange that there was a religious option for “Other” but not “No Religion,” but perhaps the pollsters simply weren’t expecting that so many people wouldn’t select the religion of their cultural tradition. I was surprised by the low proportion of Jews (several Persian American actors are Jewish), and high proportion of Zoroastrians. Of course Zoroastrians emigrated in large numbers, but their population base in Iran itself wasn’t that huge to begin with, European ethnographers were shocked to “discover” that they were still a living community in the 19th century (part of this was that they isolated themselves in remote areas to escape Muslim persecution). One hypothesis: some Iranian Americans identify as Zoroastrian to reassert their Persian cultural heritage even if they are from traditionally Muslim families (this has happened in Tajikistan somewhat, though only among intellectuals, as secularization during the Soviet period made it psychologically feasible for some to simply “leap-frog” the Muslim period back to their presumed primal identities).
Obviously Iranian Americans are very different from people in Iran. Americans sometimes assume that the anti-clerical attitude of many Iranians indicates a general anti-religious stance, but this is not evident in The World Values Survey. 16% of Iranians consider themselves “not religious” while 84% consider themselves “religious.” 0.1% were convinced atheists, out of a sample size of ~2500 (survey taken in 2005). As a comparison in Turkey 16.9% are “not religious” while in Egypt it is 7.5%. For a Middle Eastern country Iran is actually relatively on the secular side, but only for a Middle Eastern country.
Of course there is no reason that the demographics of an immigrant community would represent well that of the source region. An enormous proportion of people whose ancestors came from the Russian Empire were Jews. Arab Americans in the United States are much more likely to be Christian or other religious minorities (Casey Kasem is Druze). Until recently the rule-of-thumb has been that a majority of Arabs in the United States are Christian, not Muslim, but I suspect that is no longer true. Many people of part Arab heritage (e.g., the 1980s pop singer Tiffany) may no longer identify as Arab (and Christian Arabs seem to have had high outmarriage rates), and the recent immigrant waves have been much more Muslim in composition (this makes some sense since there simply aren’t that many Arab Christians left in the Middle East, though numerically the Copts are still substantial because of Egypt’s large base population).
A final note on Iranian Americans: did the high frequency of those with no religious affiliations emerge in the United States, or was that due to the selection biasing of the Iranian migrants? One can imagine that Iranian Communist intellectual contingent may have been irreligious, but I would assume that educated Iranians would generally have had a nominal religious affiliation to begin with. But with the option to defect in the United States, combined with a secular reaction to make themselves distinctive from the Iranian theocracy, I suspect that the generation born or raised in the United States had less use for adherence to a cultural Islam. In other words, the extent of Iranian American secularity may be as contingent as the prevalence of Latoya face & small-dog ownership among female Tehrangelinos.
(other interesting, if unsurprising, data at thePDF)