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That vote you’re about to cast may have been set in motion long ago — going all the way back to your birth and early years, when your genes and your developing brain helped determine whether you leaned conservative or liberal and how strongly you tilted that way.

Even as the presidential candidates canvas the battleground states pleading for last-minute voters to pick between them, some of the latest cutting-edge research suggests that those decisions are influenced by heritable factors that shaped those voters’ political identity long before they were presented the choice between President Obama and Mitt Romney.

“Somewhere between 20 and 50 percent of political behavior in the United States can be explained by genetics,” said James H. Fowler, professor of medical genetics and political science at the University of California at San Diego, who is one of the leading researchers in what has come to be called “genopolitics.”

Mr. Fowler said factors ranging from ideology and partisanship to likelihood of voting are influenced by genes — the only problem is that nobody knows exactly which ones and...