By Liz O’Donnell

As the search for a Supreme Court justice to replace David Souter heats up, political pundits and analysts are talking about what litmus test, if any, President Obama might apply when vetting a nominee. But scientific metaphors are not needed in this situation -- basic math skills are.

An early childhood mathematics concept called one-to-one correspondence shows us that President Obama needs to appoint a woman to the Supreme Court. Currently, 51 percent of the country's population is female. Yet only 11 percent of the Supreme Court, or one justice, is female. That is not one-to-one correspondence. With one-to-one correspondence, the members of one group can be evenly matched with the members of another.

My four-year-old daughter has mastered the concept in preschool. But our country's leaders still seem to struggle with it.