It's something of a paradox. Romantic attraction is a mysterious thing, with people in love often reporting to feeling a certain "chemistry" between them. And yet, in the great majority of cases, we end up with long-term mates who share our political ideology—a practical choice, but one that wouldn't seem to have anything to do with intensely felt, biologically based mutual allure.

Newly published research helps explain this confluence. It finds that, to a relatively small but observable degree, people are attracted to the body odor of others who share their political ideology.

That's right: To some extent, we emit red smells or blue smells, and consciously or not, potential mates can and do notice the difference.

"It appears nature stacks the deck to make politically similar partners more attractive to each other in unconscious ways," a research team led by Brown University political scientist Rose McDermott writes in theAmerican Journal of Political Science. As she and her colleagues note, this dynamic can be explained using evolutionary theory, noting that such compatibility increases the odds of successful mating and compatible child-rearing.

The study featured 146 people between the ages of 18 and 40, who were "drawn from a large city in the northeast United States." All indicated their political ideology on a seven-point scale, from "strongly liberal" to "strongly conservative."

The researchers chose 21 "target participants" who scored highly on either end of the ideological spectrum (10 liberals and 11 conservatives). After washing in fragrance-free shampoo and soap, each of them wore a gauze pad taped to each of their underarms for 24 hours. During that period, they refrained from smoking, drinking, deodorants, sex, or sleeping in a bed with another person or pet.

The samples were then frozen and stored. One week later, 125 participants "smelled each vial individually in randomized order." (Mercifully, they were treated to whiffs of peppermint in between each sample.) They rated the attractiveness of each on a five-point scale.

A pattern clearly emerged: Participants found "the smell of those who are more ideologically similar to themselves more attractive than those endorsing opposing ideologies." This suggests body odor provides a "signal of compatibility to potential mates," McDermott and her colleagues write.

"Participants never saw the individuals whose smells they were evaluating," they note. "Thus the recognition of political alignment occurred through the medium of attraction, not recognition."

On one level, this is not surprising, in that "spouses and long-term partners appear to be more similar in their political preferences than almost any other trait," the researchers write. But to understand why, they argue it's important to look at attraction from an evolutionary perspective.

"Humans, including mothers, spend most of their time around ideologically similar others," they write. "In this way, social processes may drive some of the pathways by which individuals come to prefer those whose ideological 'smell' matches their own."

"Political compatibility may also serve as a modern representation of a host of mechanisms and values that directly affect physiological and sexual compatibility, as well as child-rearing strategies," McDermott and her colleagues add. "This is because parental similarity in values increases the likelihood that such individuals may be able to say together long enough to raise their children successfully into adulthood"—which is, after all, our genes' ultimate goal.

"We do not claim that olfactory mechanisms establish an immediate or proximate cause of mate attraction, the strongest predictor of attraction, or represent the only influence on attraction," the researchers caution. "We expect that while humans are generally aware of what they are doing, and often make conscious and cognitive choices to override more basic physical desires, it is most likely that odor operates subtly and may affect the regulation of hormonal states and instigate changes in emotional mood."

In other words, body odor provides all sorts of information that people pick up on (primarily on an unconscious level) and take into account when deciding whether someone is a potentially compatible long-term mate. And those subtle clues are, to some extent, aligned with ideology, helping us find significant others of maximum compatibility.

It's yet another sign that James Carville and Mary Matalin are outliers. But then again, we already knew that.