Hackers have relatively little ability to identify emotionally with other people. This may be because hackers generally aren't much like ‘other people’. Unsurprisingly, hackers also tend towards self-absorption, intellectual arrogance, and impatience with people and tasks perceived to be wasting their time.
As cynical as hackers sometimes wax about the amount of idiocy in the world, they tend by reflex to assume that everyone is as rational, ‘cool’, and imaginative as they consider themselves. This bias often contributes to weakness in communication skills. Hackers tend to be especially poor at confrontation and negotiation.
Another weakness of the hacker personality is a perverse tendancy to attack all problems from the most technically complicated angle, just because it may mean more interesting problems to solve, or cooler toys to play with. Hackers sometimes have trouble grokking that the bubble gum and paperclip hardware fix is actually the way to go, and that they really don't need to convince the client to buy that shiny new tool they've had your eye on for two months.
Because of their passionate embrace of (what they consider to be) the Right Thing, hackers can be unfortunately intolerant and bigoted on technical issues, in marked contrast to their general spirit of camaraderie and tolerance of alternative viewpoints otherwise. Old-time ITS partisans look down on the ever-growing hordes of Unix and Linux hackers; Unix aficionados despise VMS and Windows; and hackers who are used to conventional command-line user interfaces loudly loathe mouse-and-menu based systems such as the Macintosh. Hackers who don't indulge in Usenet consider it a huge waste of time and bandwidth; fans of old adventure games such as ADVENT and Zork consider MUDs to be glorified chat systems devoid of atmosphere or interesting puzzles; hackers who are willing to devote endless hours to Usenet or MUDs consider IRC to be a real waste of time; IRCies think MUDs might be okay if there weren't all those silly puzzles in the way. And, of course, there are the perennial holy wars — EMACS vs. vi, big-endian vs. little-endian, RISC vs. CISC, etc., etc., etc. As in society at large, the intensity and duration of these debates is usually inversely proportional to the number of objective, factual arguments available to buttress any position.
As a result of all the above traits, many hackers have difficulty maintaining stable relationships. At worst, they can produce the classic geek: withdrawn, relationally incompetent, sexually frustrated, and desperately unhappy when not submerged in his or her craft. Fortunately, this extreme is far less common than mainstream folklore paints it — but almost all hackers will recognize something of themselves in the unflattering paragraphs above.
Hackers are often monumentally disorganized and sloppy about dealing with the physical world. Bills don't get paid on time, clutter piles up to incredible heights in homes and offices, and minor maintenance tasks get deferred indefinitely.
1994-95's fad behavioral disease was a syndrome called Attention Deficit Disorder (ADD), supposedly characterized by (among other things) a combination of short attention span with an ability to ‘hyperfocus’ imaginatively on interesting tasks. In 1998-1999 another syndrome that is said to overlap with many hacker traits entered popular awareness: Asperger's syndrome (AS). This disorder is also sometimes called ‘high-function autism’, though researchers are divided on whether AS is in fact a mild form of autism or a distinct syndrome with a different etiology. AS patients exhibit mild to severe deficits in interpreting facial and body-language cues and in modeling or empathizing with others' emotions. Though some AS patients exhibit mild retardation, others compensate for their deficits with high intelligence and analytical ability, and frequently seek out technical fields where problem-solving abilities are at a premium and people skills are relatively unimportant. Both syndromes are thought to relate to abnormalities in neurotransmitter chemistry, especially the brain's processing of serotonin.
Many hackers have noticed that mainstream culture has shown a tendency to pathologize and medicalize normal variations in personality, especially those variations that make life more complicated for authority figures and conformists. Thus, hackers aware of the issue tend to be among those questioning whether ADD and AS actually exist; and if so whether they are really ‘diseases’ rather than extremes of a normal genetic variation like having freckles or being able to taste DPT. In either case, they have a sneaking tendency to wonder if these syndromes are over-diagnosed and over-treated. After all, people in authority will always be inconvenienced by schoolchildren or workers or citizens who are prickly, intelligent individualists — thus, any social system that depends on authority relationships will tend to helpfully ostracize and therapize and drug such ‘abnormal’ people until they are properly docile and stupid and ‘well-socialized’.
So hackers tend to believe they have good reason for skepticism about clinical explanations of the hacker personality. That being said, most would also concede that some hacker traits coincide with indicators for non-hyperactive ADD and AS — the status of caffeine as a hacker beverage of choice may be connected to the fact that it bonds to the same neural receptors as Ritalin, the drug most commonly prescribed for ADD. It is probably true that boosters of both would find a rather higher rate of clinical ADD among hackers than the supposedly mainstream-normal 3-5% (AS is rarer at 0.4-0.5%).