For almost two centuries speculation percolated that Thomas Jefferson fathered a child by Sally Hemings, his black slave. In 1974, Fawn ­Brodie's best seller, "Thomas Jefferson: An Intimate ­Portrait," brought a level of ­respectability to the rumor—at a time when academia was showing itself increasingly open to politically correct speculation in lieu of conventional standards of proof.

Then, in 1998, Nature magazine proclaimed that new DNA evidence had settled the ­matter—that Jefferson was ­indeed the father of Hemings's...