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Vanity Fair and CBS news show 60 Minutes conducted the poll of more than 1,000 people, asking everyone to name Middleton's occupation.
From the 1,045 adults, 52% admitted that they did not know who she was.
Around 6% thought she worked as a CNBC correspondent, the same percentage as those who believed she was a children's book heroine. Another 2% thought she might be a fashion designer.
Only 32% correctly identified the younger sister of Kate Middleton as a British socialite.
Pippa was offered $5 million (£3.05 million) to appear in a porn film in May. At the time, Vivid Entertainment co-chairman Steven Hirsch said that she would only be required for one "explicit" scene.
> Photo highlights: The royal wedding
