The 8-year-old juggling a soccer ball and the 48-year-old jogging by, with Japanese lessons ringing from her earbuds, have something fundamental in common: At some level, both are wondering whether their investment of time and effort is worth it.

How good can I get? How much time will it take? Is it possible I’m a natural at this (for once)? What’s the percentage in this, exactly?

Scientists have long argued over the relative contributions of practice and native talent to the development of elite performance. This debate swings back and forth every century, it seems, but a paper in the current issue of the journal Psychological Science illustrates where the discussion now stands and hints — more tantalizingly, for people who just want to do their best — at where the research will go next.

The value-of-practice debate has reached a stalemate. In a landmark 1993 study of musicians, a research team led by K. Anders Ericsson, a psychologist now at Florida State University, found that practice time explained almost all the difference (about 80 percent) between elite performers and committed amateurs. The finding rippled quickly through the popular culture, perhaps most visibly as the apparent inspiration for the “10,000-hour rule” in Malcolm Gladwell’s best-selling “Outliers” — a rough average of the amount of practice time required for expert performance.