If you have spent any time near a radio during the past couple of months, you've probably heard a song called "Crazy," an oddball R&B ballad about insanity. With Labor Day behind us, it's safe to call the absurdly catchy tune the song of the summer.

Of course, crooning along or tapping our feet to its loping bass line, it may not occur to most of us to ask why "Crazy" — or any song for that matter —can so easily insinuate itself into our consciousness. It just sounds good, the way our favorite foods taste good.

Music as a social binding ritual is another theory that might explain its evolutionary purpose. Bringing people together to test their coordination and basic harmony, dancing and singing, could have been training for animal hunts or community defense. In fact, in many world languages one word exists for both music and dance.

People have been playing music since 34,000 B.C., but unlike other universal human activities such as eating and sex, figuring out why we love the music humans, such as the members of Trail of Dead above, create is more puzzling.

But a growing number of neuroscientists and psychologists are starting to ask exactly that question. Researchers at the Montreal Neurological Institute, for example, have scanned musicians' brains and found that the "chills" that they feel when they hear stirring passages of music result from activity in the same parts of the brain stimulated by food and sex.

As evidence mounts that we're somehow hard-wired to be musical, some thinkers are turning their attention to the next logical question: How did that come to be?

The fact is that music is universal across cultures and has been part of human life for a very long time. That archeologists have found musical instruments dating from 34,000 B.C. suggests that music may indeed be an innate human tendency. And yet it's unclear what purpose it serves.

The evolutionary benefits of our affinity for food (nutrition) and sex (procreation) are easy enough to explain, but music is trickier. It has become one of the great puzzles in the field of evolutionary psychology, a controversial discipline dedicated to determining the adaptive roots of aspects of modern behavior, from child-rearing to religion.

Some evolutionary psychologists suggest that music originated as a way for males to impress and attract females. Others see its roots in the relationship between mother and child. In a third hypothesis, music was a social adhesive, helping to forge common identity in early human communities.

In his 1997 book "How the Mind Works," Pinker called music "auditory cheesecake," a phrase that in the years since has served as a challenge to the musicologists, psychologists and neuroscientists who think otherwise.

The first modern thinker to seek a deeper purpose for music was Charles Darwin. In his 1871 book "The Descent of Man," he asserted that "musical notes and rhythm were first acquired by the male or female progenitors of mankind for the sake of charming the opposite sex."

Darwin's model was bird song. In many bird species, males sing to impress females. Depending on the species, females will tend toward the males with the broadest repertoire or the most complex or unique songs.

The foremost defender of that model today is Geoffrey Miller, an evolutionary psychologist at the University of New Mexico. Miller argues that in prehistoric communities, singing and dancing might have worked — as they do today in some American Indian cultures — as proxies for hunting and warfare. The ability to come up with imaginative melodies and rhythms would connote intelligence and creativity, and the long, arduous dances would be proof of one's endurance — the sort of traits that a choosy female would like to see in her offspring.

Miller points in particular to the example of Jimi Hendrix. Miller has written that, despite dying at 27, Hendrix had "sexual liaisons with hundreds of groupies, maintained parallel long-term relationships with at least two women, and fathered at least three children in the United States, Germany, and Sweden. Under ancestral conditions before birth control, he would have fathered many more."

Even today, he argues, the Western idea of the concert, which separates performer from audience and music from movement, is an anomaly. In many of the world's languages, Levitin points out, "there's one word for music and dance."

Indeed, if an alternate explanation is correct, it is women who were the original music-makers. One of the most universal musical forms is the lullaby.

Trehub has done research showing that mothers tend almost automatically to make their speech more musical when they talk to their babies, even more so in experiments when they are not allowed to touch them. This has led a few thinkers, Trehub included, to speculate that music may have evolved as a baby-calming tool in hunter-gatherer societies. Unlike other primate species, human babies can't simply cling to their mothers' backs, and singing may have been a way for mothers to maintain contact with their children when they had to put them down to do other tasks.

Perhaps the most widely touted explanation is that music arose as a way for groups of early humans to create a sense of community. Among other things, this might explain why music — whether it's singing hymns, school fight songs or simply "Happy Birthday" — is so often a social experience. The model is neither love song nor lullaby but anthem.

In "The Singing Neanderthal," Steven Mithen argues that communal music-making does two things. By demanding coordination and basic harmony, it works as a sort of rehearsal for the teamwork required for more high-stakes endeavors such as hunting and communal defense. And the mere act of singing and moving in time together helps forge a sense of group identity.

As evidence, Mithen, an archeologist at England's Reading University, points to the complex musical rituals of the South African Venda peopleand also to the U.S. Army, which sees chanting while marching in unison as a vital part of creating esprit de corps.

There is suggestive research linking music and sociability. Levitin, the neuroscientist, points to the difference between two mental disorders, Williams syndrome and autism. People with Williams syndrome have mental retardation, but at the same time, as Levitin puts it, are "highly social, highly verbal and highly musical." Autism, on the other hand, while it also often causes mental impairment, tends to make people both less social and less musical.

To psychologist Pinker, though, none of this adds up to a convincing case for music's evolutionary purpose. Pinker is not shy about seeing the traces of evolution in modern humans, but he stands by his "auditory cheesecake" description.

"They're completely bogus explanations because they assume what they set out to prove: that hearing plinking sounds brings the group together, or that music relieves tension," he says. "But they don't explain why. They assume as big a mystery as they solve."

Music may well be innate, he argues, but that could just as easily mean it evolved as a useless byproduct of language, which he sees as an actual adaptation.

Back in April, as part of an experiment led by Levitin to compare the physiological response of performers and listeners, Boston Pops maestro Keith Lockhart conducted the Boston Symphony Orchestra while he, a few musicians and a portion of the audience were wired with monitors that tracked their heart rate, muscle tension, respiration and other bodily signals of emotion.

"It's enough for me to know that music does have a distinct emotional reaction in almost everybody that no other art form can boast of," he says. "I've never particularly wanted to know why that happens."

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