What is Internet Addiction Disorder (IAD)?
What "Internet addiction disorder" (IAD) is still difficult to define at this time. Much of the original research was based upon the weakest type of research methodology, namely exploratory surveys with no clear hypothesis or rationale backing them. Coming from an atheoretical approach has some benefits, but also is not typically recognized as being a strong way to approach a new disorder. More recent research has expanded upon the original surveys and anecdotal case study reports. However, as I will illustrate below later, even these studies don't support the conclusions the authors claim.
The original research into this disorder began with exploratory surveys, which cannot establish causal relationships between specific behaviors and their cause. While surveys can help establish descriptions of how people feel about themselves and their behaviors, they cannot draw conclusions about whether a specific technology, such as the Internet, has actually caused those behaviors. Those conclusions that are drawn are purely speculative and subjective made by the researchers themselves. Researchers have a name for this logical fallacy, ignoring a common cause. It's one of the oldest fallacies in science, and one still regularly perpetrated in psychological research today.

Do some people have problems with spending too much time online? Sure they do. Some people also spend too much time reading, watching television, and working, and ignore family, friendships, and social activities. But do we have TV addiction disorder, book addiction, and work addiction being suggested as legitimate mental disorders in the same category as schizophrenia and depression? I think not. It's the tendency of some mental health professionals and researchers to want to label everything they see as potentially harmful with a new diagnostic category. Unfortunately, this causes more harm than it helps people. (The road to "discovering" IAD is filled with many logical fallacies, not the least of which is the confusion between cause and effect.)

What most people online who think they are addicted are probably suffering from is the desire to not want to deal with other problems in their lives. Those problems may be a mental disorder (depression, anxiety, etc.), a serious health problem or disability, or a relationship problem. It is no different than turning on the TV so you won't have to talk to your spouse, or going "out with the boys" for a few drinks so you don't have to spend time at home. Nothing is different except the modality.

What some very few people who spend time online without any other problems present may suffer from is compulsive over-use. Compulsive behaviors, however, are already covered by existing diagnostic categories and treatment would be similar. It's not the technology (whether it be the Internet, a book, the telephone, or the television) that is important or addicting -- it's the behavior. And behaviors are easily treatable by traditional cognitive-behavior techniques in psychotherapy.

Case studies, the alternative to surveys used for many conclusions drawn about online overuse, are just as problematic. How can we really draw any reasonable conclusions about millions of people online based upon one or two case studies? Yet media stories, and some researchers, covering this issue usually use a case study to help "illustrate" the problem. All a case study does is influence our emotional reactions to the issue; it does nothing to help us further understand the actual problem and the many potential explanations for it. Case studies on an issue like this are usually a red flag that help frame the issue in an emotional light, leaving hard, scientific data out of the picture. It is a common diversionary tactic.

There is more research that needs to be critically examined here, which I will provide descriptive analyses of shortly.

Why Does the Research Leave Something to Be Desired?
Well, the obvious answer is that many of the original researchers into the phenomenon known as IAD were actually clinicians who decided to conduct a survey. Usually doctoral training is sufficient to create and test a survey, yet the psychometric properties of these surveys are never released. (Perhaps because they were never conducted in the first place? We simply do not know.)
The obvious confounds are never controlled for in most of these surveys. Questions about pre-existing or a history of mental disorders (e.g., depression, anxiety), health problems or disabilities, or relationship problems are absent from these surveys. Since this is one of the most obvious alternative explanations for some of the data being obtained (for example, see Storm King's article, Is the Internet Addictive, or Are Addicts Using the Internet? below), it is very surprising these questions are left off. It taints all the data and make the data virtually useless.

Other factors are simply not controlled for. The current Internet population is nearly 50/50 in terms of proportion of men to women. Yet people are still drawing conclusions about this same group of people based upon survey samples that have 70-80% men, comprised mostly of white Americans. Researchers barely mention these discrepancies, all of which will again skew the results.

Research done in a particular area should also agree about certain very basic things after a time. Years have gone by and there are more than a few studies out there looking at Internet addiction. Yet none of them agree on a single definition for this problem, and all of them vary widely in their reported results of how much time an "addict" spends online. If they can't even get these basics down, it is not surprising the research quality still suffers.

More research has been done since the original surveys were released in 1996. This newer research has been conducted by more independent researchers with clearer hypotheses and stronger, less biased population sets. More about these studies will be discussed in updates to this article.

Where Did It Come From?
Good question. It came from, believe it or not, the criteria for pathological gambling, a single, anti-social behavior that has very little social redeeming value. Researchers in this area believe they can simply copy this criteria and apply it to the hundreds of behaviors carried out everyday on the Internet, a largely pro-social, interactive, and information-driven medium. Do these two dissimilar areas have much in common beyond their face value? I don't see it.
I don't know of any other disorder currently being researched where the researchers, showing all the originality of a trash romance novel writer, simply "borrowed" the diagnostic symptom criteria for an unrelated disorder, made a few changes, and declared the existence of a new disorder. If this sounds absurd, it's because it is.

And this speaks to the larger problem these researchers grapple with... Most have no theory driving their assumptions (see Walther, 1999 for a further discussion of this issue). They see a client in pain (and in fact, I've sat in many presentations by these clinicians where they start it off with just such an example), and figure, "Hey, the Internet caused this pain. I'm going to go out and study what makes this possible on the Internet." There's no theory (well, sometimes there's theory after-the-fact), and while some quasi-theoretical explanations are slowly emerging, it is putting the chicken far before the egg.

Do You Spend Too Much Time Online?
In relation to what or whom? Time alone cannot be an indicator of being addicted or engaging in compulsive behavior. Time must be taken in context with other factors, such as whether you're a college student (who, as a whole, proportionally spend a greater amount of time online), whether it's a part of your job, whether you have any pre-existing conditions (such as another mental disorder; a person with depression is more likely to spend more time online than someone who doesn't, for instance, often in a virtual support group environment), whether you have problems or issues in your life which may be causing you to spend more time online (e.g., using it to "get away" from life's problems, a bad marriage, difficult social relations), etc. So talking about whether you spend too much time online without this important context is useless.
What Makes the Internet So Addictive?
Well, as I have shown above, the research is exploratory at this time, so suppositions such as what makes the Internet so "addictive" are no better than guesses. Since other researchers online have made their guesses known, here are mine.
Since the aspects of the Internet where people are spending the greatest amount of time online have to do with social interactions, it would appear that socialization is what makes the Internet so "addicting." That's right -- plain old hanging out with other people and talking with them. Whether it's via e-mail, a discussion forum, chat, or a game online (such as a MUD), people are spending this time exchanging information, support, and chit-chat with other people like themselves.

Would we ever characterize any time spent in the real world with friends as "addicting?" Of course not. Teenagers talk on the phone for hours on end, with people they see everyday! Do we say they are addicted to the telephone? Of course not. People lose hours at a time, immersed in a book, ignoring friends and family, and often not even picking up the phone when it rings. Do we say they are addicted to the book? Of course not. If some clinicians and researchers are now going to start defining addiction as social interactions, then every real-world social relationship I have is an addictive one.

Socializing -- talking -- is a very "addictive" behavior, if one applies the same criteria to it as researchers looking at Internet addiction do. Does the fact that we're now socializing with the help of some technology (can you say, "telephone"?) change the basic process of socialization? Perhaps, a bit. But not so significantly as to warrant a disorder. Checking e-mail, as Greenfield claims, is not the same as pulling a slot-machine's handle. One is social seeking behavior, the other is reward seeking behavior. They are two very different things, as any behaviorist will tell you. It's too bad the researchers can't make this differentiation, because it shows a significant lack of understanding of basic behavioral theory.

Alternative Hypotheses
In addition to those previously discussed, here is an alternative hypothesis that no research to date has seriously considered -- that the behaviors we are observing are phasic. That is, for most people with "Internet addiction," they are likely newcomers to the Internet. They are going through the first stage of acclimating themselves to a new environment -- by fully immersing themselves in it. Since this environment is so much larger than anything we've ever seen before, some people get "stuck" in the acclimation ( or enchantment) stage for a longer period of time than is typical for acclimating to new technologies, products, or services. Walther (1999) made a similar observation based upon the work of Roberts, Smith, and Pollack (1996). The Roberts et al. study found that online chat activity was phasic -- people first were enchanted by the activity (characterized by some as obsession), followed by disillusionment with chatting and a decline in usage, and then a balance was reached where the level of chat activity normalized.
I hypothesize that this type of model can be more globally applied to online usage in general:

Some people simply get caught in Stage I and never move beyond it. They may need some help to get to Stage III.
For existing online users, my model allows for overuse as well, since the overuse is defined by finding a new online activity. I would argue, though, that existing users have a much more easier time successfully navigating through these stages for new activities they find online than newcomers to the Internet. It is possible, however, for an existing user to find a new activity (such as an attractive chat room or newsgroup or Website) that could lead them back into this model.

Note one important distinction about my model... It makes the assumption that since all online activity is phasic to some degree, all people will eventually get to Stage III on their own. Just like a teenager learns to not spend hours on the telephone every night on their own (eventually!), most adults online will also learn how to responsibly integrate the Internet into their lives. For some, this integration simply takes longer than others.

What Do I Do If I Think I Have It?
First, don't panic. Second, just because there is a debate about the validity of this diagnostic category amongst professionals doesn't mean there isn't help for it. In fact, as I mentioned earlier, help is readily available for this problem without needing to create all this hoopla about a new diagnosis.
If you have a life problem, or are grappling with a disorder such as depression, seek professional treatment for it. Once you admit and address the problem, other pieces of your life will fall back into place.

Psychologists have studied compulsive behaviors and their treatments for years now, and nearly any well-trained mental health professional will be able to help you learn to slowly curve the time spent online, and address the problems or concerns in your life that may have contributed to your online overuse, or were caused by it. No need for a specialist or an online support group.