Some chess players are devastated by losses, letting them infect their mood and their subsequent play. Losing at chess does not bother him that much, Magnus said. “I get more upset at losing at other things than chess. I always get upset when I lose at Monopoly.” So much so that his sisters team up to beat him.

Their fervent sibling rivalry is the rare blot on a home life that seems almost idyllic. His father, Henrik, is an information technology consultant who has spent a good deal of time traveling with his son when not working; his mother, Sigrun Carlsen, is a chemical engineer. Both are active and down to earth. Magnus has three sisters: Ellen, 19; Ingrid, 14; and Signe, 11. Ingrid said her brother often teases her, almost to the point of bullying, something that he admitted with a devilish grin. “What kind of brother would I be if I didn’t tease her?” he asked.

Stories of chess prodigies often begin with a eureka moment when the child discovered the game or displayed some heretofore hidden talent for it. Magnus’s interest can be traced to competition with his sister Ellen. Their father, a good tournament-level player, tried to teach Magnus and Ellen to play when they were 5 and 6, but neither showed much interest. It was three years later, after Ellen had begun to play seriously, that Magnus started learning the game, mostly because he wanted to beat her.

One day, when he was 8, he challenged Ellen to a game and won. “The sad thing about that,” their father said, “was that she then quit for four years.”

The boy, meanwhile, was hooked. He began playing regularly in tournaments, and his progress was, by chess measures, spectacular. A year after playing his first tournament against children his own age, he was competing against adults and beating them. For a while he studied with Simen Agdestein, a grandmaster and former Norwegian champion, but Magnus has now surpassed him and studies on his own to supplement the vast catalog of positions he has memorized. Last year he spent more than 200 days on the road playing and earned $250,000 after expenses, his father said. Given his results this year, his earnings will surpass that.

Image Magnus in the Dubai Open tournament in 2004. Credit... Rabih Moghrabi/Agence France-Presse

Henrick Carlsen said his son’s progress in chess was typical for him. “Sometimes, he’s been thought to be slow,” he said. But when he gets interested in something, “then he accelerates.” He added, “I don’t think he is conscious of this approach. It is innate.” He said that his son, from a young age, exhibited an ability to focus single-mindedly. One day, the father recalled, when Magnus was 4, he spent six hours building a train out of Legos. A half-hour after he went to bed, Mr. Carlsen found him in the dark “wide awake and staring into space, and I thought, ‘O.K., this is too much.’ ”