Watch the 18-year-old music prodigy William McGregor play a classical concerto on his double bass – his expressive face, framed by long wavy blond hair, rising in emotion along with the music – and it's hard not to wonder one thing.

How could this Malvern teenager, just named one of only 20 U.S. Presidential Scholars in the Arts from over 5,000 candidates, have started learning to master an instrument 6 feet tall with the bulk of a linebacker when he was only 2 years old?

When McGregor's parents started the then-toddler for music lessons in his native Michigan, his older brother Colin was already learning the violin. The parents wanted William to learn something different, and Colin's teacher's husband, a professional bass player, had an idea.

"He always wanted to try teaching a very young student" to play bass, said William McGregor. "I was his guinea pig." He took a 3-foot-tall cello, which still seemed "ginormous" to his 2-year-old student, and tuned it to make it sound like a bass, albeit a higher-pitched one.

McGregor proved much more than a successful guinea pig. As he grew into his full-size stand-up bass, he also grew into one of the top young classical musicians in the country. His selection this month as the only Presidential Scholar in the Arts from Pennsylvania comes a year after the Juilliard-trained musician won the prestigious Stulberg International String Competition, only the second time a double bass player has won in 42 years.

"He plays well beyond his years, both technically and … with great maturity, sensitivity and emotion," said Margaret Hamilton, head of the Kalamazoo, Mich.-based competition, which comes with a $6,000 grand prize. "He's a delightful young man."

McGregor's teenage virtuosity invokes the age-old question that still bedevils 21st century parents: Is such a one-of-a-kind talent the result of nature or nurture?

William's mother, Nancy, who decided while she was pregnant with Colin to immerse her children in the Suzuki method for teaching very young musicians, said she's confident music wasn't in her son's genes. "I played bad piano when I was younger," she noted, "and my husband didn't play anything."

But thanks to that early start, William's very first memory is of playing that three-foot adapted cello in his living room. "I didn't know what I was doing," he said, "but I just enjoyed playing."

Now, as he prepares to graduate PA Leadership Charter School near West Chester and begin studies at Philadelphia's renowned Curtis Institute of Music, one of the most striking things about McGregor is that a kid who's spent thousands of hours practicing his art – including traveling to Juilliard's pre-college program almost every Saturday for nine years – has been able to hold onto his everyday boyish obsession with sports, especially baseball cards.

Indeed, for an ex-Little Leaguer who is obsessed with the art of pitching, baseball has proved as much an outlet for a kid brimming with energy as the physicality of playing the double bass. "They used to roll up paper into a baseball and play at Juilliard," his mom recalled. "It would drive the parents crazy."

When William was 5, he bought a pack of basketball cards at a Kmart and found his then-idol, Rip Hamilton of the Detroit Pistons. He was hooked. "I was just like, wow, you can get your favorite player!" he recalled.

Soon, he was going to garage sales and flea markets where he could buy 400 baseball cards for a dollar in hope of finding one of his beloved Detroit Tigers. Today, McGregor is more likely to look for baseball cards that he can sell – and he made roughly $2,000 last summer.

The hobby also proved a way to bond with his brother, now a 20-year-old studying cognitive science at Johns Hopkins University. Colin had developed his own passion for collecting rare license plates, and so three or four days a week every summer the two rise at 4 in the morning to hit flea markets in rural Pennsylvania where they sell their collectibles.

Boxes of William's baseball cards are stacked in his bedroom and the basement at their home. The family, including his dad, Brian, a regional manager for Bed Bath & Beyond, moved there from Ann Arbor when William was 9. It brought them closer to Juilliard, a 10-hour drive from Michigan. Collecting is still just a hobby compared with playing the double bass.

"I can't remember a time in my life when I never wanted to do it," he said. Added his mother, playing bass "is when he's the happiest."

While brother Colin never pursued the violin with William's musical passion, he said the brothers sometimes end up "screwing around to '80s rock songs on violin and bass, [and] playing the Killers in our living room" when he's home for summer break.

But he might be William's biggest fan and is serious when he says his kid brother is "going to go down as perhaps the greatest bassist of his generation."

William's long-time teacher at Juilliard, Albert Laszlo, said he's never known a bassist who started on the instrument so young, and thinks that contributed to his success. But he also credited his "incredible abilities," citing his skill on the fingerboard as well as hard work. "He would practice consistently year in and year out and do all the assignments I gave him incredibly thoroughly," Laszlo said.

Then there's his love of performing, which gives him "tremendous freedom to communicate emotion with the instrument, which is why he wins all these competitions," the teacher said.

As much as he loves classical music, William confesses to liking "teenage girl" bands like Maroon 5, Green Day, and Muse, and he also hopes that someday he'll be performing on the popular hit recordings.

"My dream job would be playing studio work," he said. "I love the bass, and I know how diverse it is. As much as I love classical genre, it's quite limited and very competitive. If you brand yourself to play pop and rock, the opportunities expand quite a bit."

Yet McGregor's favorite piece is still the Moses Variation by Paganini, the Italian violinist, and he won the Stulberg competition by playing Bottesini Concerto No. 2 – the kind of classical work that he began performing in public with youth orchestras or small recitals since he was 3 or 4. The years of dedication and practice paid off when he overcame a rough bout with Lyme disease that had laid him low right before the contest.

"For the bass, it's sort of the pinnacle piece," he said of the Bottesini work. "It's the one if you're a double bassist that's the hardest and most challenging one you want to play." The Stulberg award came not just with a cash prize but a chance for McGregor to perform with top symphonies in Kalamazoo, Baltimore, and Grand Rapids, Mich. And as a presidential scholar, he gets to play next month in Washington at the Kennedy Center for the Performing Arts with his fellow honorees.

By then, he'll be preparing to study at Curtis, which he chose over other options such as Juilliard and the University of Michigan for a chance to learn from the world's top bass instructor, Harold Robinson. It's all been a remarkable payoff on an investment that began when McGregor could barely walk.

Nancy McGregor says William was free to put down his bow at any time. He just never wanted to. "If you start them at 2, it's a part of who they are," she mused, "so they never know what it's like to not have it. …They didn't understand you can rebel."