Both Uses of

mutual fund
in
Freakonomics

One group of mutual fund companies let preferred customers trade at preferred prices, and another group was charged with hiding management fees.†
p. 66.1 *

mutual fund = a fund that enables investors to pool their money and place it under professional investment management

But as David Hillis, an interventional cardiologist at the University of Texas Southwestern Medical Center in Dallas, explained to The New York Times, a doctor may have the same economic incentives as a car salesman or a funeral director or a mutual fund manager: "If you're an invasive cardiologist and Joe Smith, the local internist, is sending you patients, and if you tell them they don't need the procedure, pretty soon Joe Smith doesn't send patients anymore."†
p. 67.7

Definitions:

(1) (mutual fund)
a fund that enables investors to pool their money and place it under professional investment management
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus