I was going to argue with Lisa about this but then I decided she's closer to the truth than I was.  The issue isn't whether the rent is below market, but whether there is profit motive.  I have a client who is renting a house to a family member at lower than market rates.  They were going to sell it a couple years ago, but decided that even vacant real estate was a better investment than Bitcoin or GameStop.  Since then, the property value has increased by at least 50%.  They don't want to deal with a stranger as a tenant, and the tenant/relative is a potential commission-free buyer.  There is no mortgage, so they still show a profit on Schedule E.  

Is it possible to have the other side of the coin -- fair rental value, but no profit motive?  Suppose someone knows they can make 20% in the market but choose to rent to a kid with a good job but no credit and recently released from prison, even though the return is only 5% and real estate prices are declining.  The case could be made that this shouldn't be on Schedule E, but it's unlikely that IRS will be making it anytime soon.