North has it right - for a non-margin account, meaning margin is not enabled, not that you have it but aren't using it, a broker may not loan your shares out w/o getting your expressed permission. If you "borrowed" your neighbors car w/o asking him first you'd get arrested. Nothing can stop your broker from lending shares of any stock you own in an account with margin enabled, it's in the fine print - meaning if you own AMRN you bought with cash, and IBM you bought with cash, they can loan out either stock to shorts. Most brokers share a % of the loan fees, but it's pitifully small in some cases.
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