Barack Obama: The Story, by David Maraniss, Simon & Schuster, 672 pp., $32.50.

The first stories to emerge from pre-release copies of Barack Obama: The Story, David Maraniss' new biography of the president, have centered on Obama's youthful enthusiasm for marijuana. As a scholarship student at Hawaii's exclusive Punahou prep school, Obama embraced pot-smoking with his friends in the "Choom Gang." He was even competitive about it, preaching "total absorption" (hold the smoke in so that little or none comes out when you exhale) and calling "Interception!" when he grabbed a joint to take a hit out of turn.

How do you square Obama's youthful passion for pot (he continued smoking it well into college, only to phase it out, more or less, after graduating) with the Obama administration's aggressive enforcement of federal anti-pot laws in states where medical marijuana has been legalized? The president who acknowledged his youthful drug use in his 1995 memoir Dreams from My Father ("I had learned not to care. Pot had helped, and booze; maybe a little blow when you could afford it") offered Rolling Stone a lame justification for heightened enforcement on his watch earlier this year: "I can't nullify congressional law. I can't ask the Justice Department to say, 'Ignore completely a federal law that's on the books.' What I can say is, 'Use your prosecutorial discretion and properly prioritize your resources to go after things that are really doing folks damage.' As a consequence, there haven't been prosecutions of users of marijuana for medical purposes."

Never mind that prosecutorial discretion just is "ignoring" a law in order to focus on what's really doing damage. And never mind that neither marijuana nor the pot-growing industry—nor even "a little blow"—seems to have done any noticeable damage to Obama. Penn Jillette spoke for many when he declared Obama a hypocrite on drug policy.

But is Obama a hypocrite? A liar? A strategically adept politician trying to immunize himself from charges that he's soft on drugs? Is he simply waiting for the right moment to tell us his position on marijuana has "evolved," running just a bit behind his evolved position on same-sex marriage? Maraniss' book doesn't try to answer these vexing current questions, but it does give readers several clues why the youthful, exuberant pothead Barry Obama may have tolerated or even encouraged federal anti-marijuana enforcement on his watch.

The keys to understanding Obama, Maraniss insists, lie in his family background, his youth as a dislocated, fatherless, biracial son of an American anthropologist mother with the unlikely maiden name of Stanley Ann Dunham. Nearly every factor in Obama's upbringing, from his childhood years in Indonesia to his adolescence in Hawaii and to his college and post-college intervals in California, New York, and Chicago, seems breathtakingly random. Knowing that his absentee biological father was a brilliant and charismatic (though increasingly alcoholic and self-destructive) Kenyan bureaucrat further underscored how fundamentally different the young Obama felt himself to be. Native to no culture, he found himself self-consciously crafting his identity.

For all its randomness, however, Obama's youthful experience could not have been better designed to reinforce in the young man something like his mother's anthropological observational distance mixed with empathy, packaged in the self-crafted "cool" even-tempered islander persona we have seen in Obama as an adult. (More than once Maraniss reminds us of a Hawaiian pidgin maxim: "Cool head, main thing.")

We also see the roots of Obama's immensely cautious temperament, which played out in his early personal relationships (an ex-girlfriend's treasure trove of a diary features recurrent complaints about his "withholding the kind of emotional involvement that I was seeking") and in his early professional activism, hired by a Catholic-sponsored organization to work as a community organizer in Chicago. The man who picked Obama for that job, Jerry Kellman, describes Obama, then in his mid-twenties, as "one of the most cautious people I've ever met in my life." The future president, Kellman says, "was not unwilling to take risks, but was just this strange combination of someone who would have to weigh everything to death, and then take a dramatic risk at the end. He ws reluctant to do confrontation, to push the other side because it might blow up—and it might. But one thing [organizing guru Saul] Alinsky did understand was that within reason, once something blows up, to a certain degree it doesn't hurt, it helps. You are supposed to be cautious about taking on issues you can't win, but Obama was more cautious than you need to be." Once he did enter a fight, though, "he was willing to take a very significant risk."

Maraniss implicitly agrees with Kellman. Chicago, he writes, "was the day-to-day world of urban black America, a place that for all of his travels [e.g., his years in California and in New York City] he had never really experienced before." Obama's persistent sense of his own disconnectedness (in race, in culture, in family) had taught him to look before he leapt. But the process of engaging directly with South Side communities for once broke through his temperamental caution, and he joined a Chicago church.

Chicago was also the real genesis of Obama's commitment to politics, and to law school as preparation for a political career. Obama tells Maraniss in a White House interview that a group event he participated in with Chicago mayor Harold Washington put things in perspective: "[Washington] comes out and they were all looking at him starry-eyed, and he was making jokes and flirting with the women and…you just had this sense that his ability to move people and set an agenda was always going to be superior to anything I could organize at a local level." Maraniss comments: "Here was the charisma of power. Power that was larger, more encompassing, and that could come only from politics and established institutions, power that demanded a more granular understanding of how the world worked." Soon after, Maraniss writes, Obama was applying to Harvard Law School.

Barack Obama: The Story clocks in at 672 pages, and scarcely any of those pages addresses Obama's public record as we now know it. It concludes with the young Obama completing his visit to the Africa of his father and grandfather (and a raft of Obama relatives), then beginning his journey from his adopted home Chicago to attend Harvard Law. But the temperamental roots of Obama's deliberative cautiousness as president, his search for and encouragement of consensus, and even his apparent hypocrisy on marijuana policy, are all here. Maraniss' near-obsessive interest in Obama's family tree can be taxing. (A reader may doubt the relevance of a great-grandparent's suicide.) But his nearer-term focus on Obama's youth—essentially characterizing Obama as a lifelong outsider whose racial and cultural identity was chosen rather than inherited—does much to explain the president's incrementalist approach to some issues (homosexual rights, winding down involvement in Iraq) and his conservative unwillingness to rock the boat on others (marijuana). It will be interesting to see how well this socially adept yet introverted, cerebral, and consensus-seeking outsider competes against a similarly inward-directed, private, and compromising—yet otherwise very different—outsider in the fall.

Mike Godwin is a contributing editor at Reason magazine.