And a great cry arose from all across the land: in business offices where managers flashed back to important details they had missed during meetings; in cubicles where wage slaves recalled the countless hours lost trying to figure out what they were doing just before the interruption; at power-lunch spots from Manhattan to Malibu where patrons mourned the relationships they had sundered with their habit; in homes where spouses seethed over the third party in their bed; in labs where scientists studied the impact of technology on thinking; on train platforms where commuters wistfully recalled the days when they spent their wait mulling, pondering and daydreaming. That cry, uttered in response to the news that President Obama was getting to keep his beloved BlackBerry, sounded like this:

Uh-oh.

Technology has affected the way people think, interact and make decisions ever since Homo erectus mastered fire and, for the first time, had a way to keep surplus mammoth from rotting. Result: cooperative hunting. The cognitive and social effects of the BlackBerry on its 21 million users aren't so unambiguously beneficial. So while legions of BlackBerry fans cheer Obama's success in keeping his, insisting it makes users more productive and connected, experts in cognitive psychology and in human-machine interactions who study pop-ups, e-mail alerts, calendar reminders and instant messaging—the most intrusive and ubiquitous pre-BlackBerry technologies—have two things to say: distraction overload, and continuous partial attention. For whatever the virtues of a handheld, there is no question that, depending how you use it, you risk never focusing exclusively on any thought or perception for long and never being able to work straight through to completion on anything. That's OK for tasks you can handle with half your cerebral lobes tied behind your back. It's less fine when the task is, say, watching for track signals while operating a train.

How damaging an interruption is depends on when it occurs. In a 2004 study, scientists led by Brian Bailey of the University of Illinois had volunteers edit text and search the Internet while being interrupted by news alerts. It was much less annoying to be interrupted between what the scientists call "coarse breakpoints," such as at the completion of a paragraph or thought. Not only is it easier to jump back into the previous task after the interruption, but when you are not trying to keep in your head what you need to finish a task you can pay more attention to the interruption itself. If you answer the BlackBerry's call at natural breakpoints, you're much more likely to be able to take in the e-mail and then resume what you were doing without that "where was I?" brain lock. In some demanding tasks, however, there may not be any natural breakpoints. Pilots who are interrupted during a preflight checklist sometimes miss an item when they try to pick up where they left off, notes psychologist Deborah Boehm-Davis of George Mason University, who studies interruptions. Last summer's crash of an airliner taking off from Madrid was apparently the result of an interruption-induced error; 153 people died.

There are no good studies on how often BlackBerry users let themselves be interrupted by its seductive call, but research on other electronic interruptions is not encouraging. When Gloria Mark, professor of informatics at the University of California, Irvine, shadowed employees at two high-tech firms, she found that the average worker spends only 11 minutes on any given project before being interrupted and asked to do something else. IT workers have it worse, switching attention every three minutes, on average. The BlackBerry (which isn't nicknamed CrackBerry for nothing) is way more seductive than, say, e-mail alerts. Thanks to its growing social and cultural cachet, it can make the most inconsequential middle manager feel as important as the CEO who must always be reachable, and it can feed the illusion of the lowliest salaryman that his input is so central he must be thumbing away at the dinner table and on vacation.

The distraction of almost-irresistible interruptions has been the killer app, so to speak, for some BlackBerry newbies. Sen. Thad Cochran of Mississippi, like almost every politician on the Hill, got a BlackBerry after 9/11 for security purposes. But he gave it back. "I was always distracted," Cochran said. "I couldn't concentrate. Every time the light came on or it beeped, I felt this compulsion to stop everything I was doing." Though he doesn't begrudge his colleagues for their addiction, Cochran says the result is that during meetings on the Hill almost everyone is "always checking messages" or typing, he says. "It just beeps or buzzes, all the time, and people get up and leave the room."

Interruption overload can impair higher cognitive functions, too, starting with decision making. It takes time to bring your mind back to the task you left when the BlackBerry called, which means (if that task was listening to someone, for instance) you have missed more than occurred during just the seconds it took to read an e-mail. People take about 15 minutes to productively resume a challenging task when they are interrupted even by something as innocuous as an e-mail alert, scientists at Microsoft Research and the University of Illinois found in a 2007 study. The delay may reflect how long it takes to reactivate memories about the task and to "refocus cognitive resources that may have been usurped" by the interruption, they reported. If this delay causes you to miss key information, then you are basing a decision on incomplete knowledge, something even BlackBerry lovers understand. Sen. Robert Menendez of New Jersey, for instance, was spotted last year typing on his BlackBerry as he gaveled the debate on the Senate floor. "I've got a lot of traffic and action going on, and it helps me keep track of it all," he said. But he turns it off during crucial hearings and meetings, recognizing that it impairs his concentration and could make him miss key information.

Interruptions can also derail brain processes that sort incoming signals. Information first lands in short-term memory, but if it is to stay with you for the long term it must be encoded—put in the right mental file drawer. That process requires a few minutes, and, if interrupted, can be short-circuited. "Being forced to divert attention to interrupting messages," scientists in Finland concluded in a 2004 study, "can cause memory loss" and "decreased memory accuracy." If new information is not indexed correctly, some of what the brain has stored about, say, TARP will be inaccessible; it's there, but you've failed to construct the neuronal road map needed to find it.

The more brain power an interruption demands, the more disruptive it will be to the task it is pulling you away from. If dealing with the interruption requires so little concentration that you are still able to unconsciously "rehearse" the task you broke away from, be it programming your TiVo or walking into a room to retrieve something, you will do a better job on that task when you return to it. If the interruption requires significant mental effort, however, rehearsal breaks down and the subconscious cannot keep repeating, "I walked into this room to get my wallet." Hence the feeling of "What'd I come in here for?!"

Continuous partial attention is actually a misnomer. Computer scientists use it, but most psychologists disdain it because what seems like partial attention or multitasking is actually rapid-fire switching of attention among tasks. In that state of mind, says computer scientist Mary Czerwinski of Microsoft Research, you don't process information as fully and are not using your frontal lobe effectively.

A BlackBerry can have detrimental effects even, or especially, when users turn it on when they are doing "nothing." That "nothing" is what our pre-BlackBerry forebears called daydreaming, which is a propitious mental state for creativity, insight and problem solving. Truly novel solutions and ideas emerge when the brain brings together unrelated facts and thoughts. That is hard to achieve when you are attacking the problem head on. Because the idea of "buying music" is associated with specific thoughts that you have thought every time you've pondered that act, thinking directly about it sends brain signals along the same well-worn neuronal roads, arriving at the same oft-visited nodes. But daydreaming or thinking about something else keeps the signals off those rutted roads and allows far-flung facts and ideas to combine in novel ways, producing, say, iTunes. Hence the common experience of an "aha" moment of creativity or insight about some problem when it is not commanding your conscious attention. If mental downtime becomes BlackBerry time, eurekas will be rarer.

When Teresa Amabile of Harvard Business School studied 238 people working on projects that required creative solutions, she found that fragmentation of attention also impeded creativity. Time pressure typically had the same effect, unless attention was focused on a crucial problem. (A surge of adrenaline can ramp up mental processes.) The NASA engineers who came up with a duct-tape-and-spit solution to Apollo 13's crisis in 1970 faced crushing time pressure—the three astronauts would die if NASA did not find a way to filter out carbon dioxide in the air, reconfigure power use and put the spacecraft on a new return trajectory within days. But they had no distractions (the brass at Mission Control shielded them from all interruptions) and their attention was totally focused, as thoughts of death tend to do.

Given the damage caused by interruption overload and continuous partial attention, we can infer either of two things about people who use their BlackBerry while holding a conversation, weighing decisions, trying to solve a problem or attempting to do creative work with, they claim, no ill effects. Possibility one: they are lying. Possibility two: their work just isn't that hard. Yes, you can schedule meetings. No, you cannot craft a smart stimulus bill. One wonders whether financiers who did not comprehend the esoteric derivatives they were selling (as Robert Rubin, former co-chairman of Citigroup, told NEWSWEEK last year) might have understood them if they had given the guys who invented them their full and undivided attention, rather than BlackBerrying during presentations. But we're just guessing here.

Obama will be spared the worst consequences of continuous partial attention and interruption overload for the simple reason that he agreed to cut way back on his BlackBerry habit, using it mainly to stay in touch with family, close friends and top advisers. Although the White House declined to say when and where he will use his BlackBerry or even have it on, sources say it is off during briefings and meetings. Several lawmakers who have met with Obama privately in recent weeks tell NEWSWEEK they haven't seen him with it—"and I was looking," said one Republican senator.

BlackBerry fans who cheered Obama's victory over those who would take it away from him assume that the leader of the free world will be shielded from the damaging cognitive effects it can have on mere mortals. Roger McNamee, managing director of the Silicon Valley venture-capital firm Elevation Partners, had nothing but scorn for the suggestion that a BlackBerry might pose problems in the Oval Office. "You just turn notification off—that's what everybody does—and it doesn't bother you at all," he told NEWSWEEK's Daniel Lyons. "I do not allow notifications for e-mail, text or any other application. As a result, the devices are not a distraction." Even Senator Menendez, who recognizes that an insistent BlackBerry is not something you want when witnesses at a hearing are trying to explain the arcane details of, for example, toxic bank assets, says, "The bottom line is, I couldn't imagine accomplishing all the things I need to do without it." (Viacom's Sumner Redstone and News Corp.'s Rupert Murdoch, however, apparently have no trouble imagining it: neither is known to use a handheld.)

In Obama's case, however, it is hard to see how the man who is never farther from an aide than his pillow is to the bedroom door is in any danger of being unconnected from the people and information he needs for his job. "He's not getting important facts on it—he's probably staying in touch with his buddies," speculates Rep. Anthony Weiner of New York, himself an admitted BlackBerry addict. "I'm sure he's much more likely to glance down to see how the Steelers are doing and stay connected than to use it for work. I'm not concerned that the president will be distracted. But the next time he's at a state dinner and glances down under the table, he's much more likely to be BlackBerry- ing his wife 'When can I leave' than anything official."

At least Obama shouldn't have to worry about one detrimental effect of BlackBerry interruptions. The stress and hence the cognitive damage caused by e-mail, text and similar intrusions are inversely related to a person's self- esteem and to how much control he perceives he has over his working environment, scientists in Scotland reported in 2006. Obama has not shown many signs of low self-esteem. Similarly, people who feel they are at the whim of individuals and forces beyond their control tend to suffer the worst consequences of interruption overload. Lowly personal assistants who are constantly juggling the boss's BlackBerry texts and e-mails, terrified of missing an important one, have the most trouble returning to what they were doing before the BlackBerry trilled, and suffer the worst cognitive lapses. If you're the most powerful man in the world? Not a worry.

With Holly Bailey and Daniel Stone in Washington and Jeneen Interlandi in New York