As far as the difference between following a trend and possibly having a mental illness goes..
I do hate those who mimick actual, serious problems because they want attention. A few years back, some girls in my grade started cutting because they thought it was the "in" thing to do. It only happened a few times; they were turned in to the guidance office, questioned, and that was that. It went no further (obviously, they realized they were dabbling in a serious matter and stopped their phony ways). I was so aggravated by what they were doing that I wrote a poem filled with conviction and shared in in front of our English class. It was a called A Message to the Sane Liars. I got a standing ovation, if that tells you anything.
The ironic part about this is that I didn't start harming myself for another two years after that incident. At the time, I did think it was a bit pointless, but those "cutters" had their way of "dealing with things". So I spared them the criticism. It really pissed me off when I discovered that girls in my class were doing it simply for the attention. For the fad. Oooh, I wanted to hurt them myself. I hate posers.
However. And, this is a big however.
Harming yourself simply for acceptance is disturbing, yes. But what happens when that behavior is taken to an extreme? Isn't it possible that those who harm themselves to "fit in" may suffer from their own mental disorder? I bring this up because I know a girl who began cutting to "fit in". Now, I suppose she continues to do it out of habit (for those of you who are experienced, you know how addicting it can get). The thing about this girl is, there's something wrong with her. Like, seriously wrong. She is addicted to seeking acceptance. It's so sickening that I believe it verges on a disorder. When I tell her about bands that I listen to, she obsesses over them. I don't really even know if she likes them, or if she is simply doing it to win my approval. She prints out pictures and lyrics from the band and pastes them in a collage on her bedroom walls. She did this once, literally hours after I told her about my new favorite band. It annoyed me to no end. My best friend has a bladder problem; she's had it for the near-decade that I've known her, and let me tell you, she would do anything to get rid of it. It's a burden for her, and a physical problem. This girl that I speak of (the one who has an extreme longing for acceptance) went so far as to proclaim that she has the same bladder disorder as my friend. "If I laugh too hard, I pee my pants. I can't help it I have a weak bladder." That is exactly what she said to me. I was offended for my friend. That was just ridiculous.
My point: as you can see, there lies a deeper question when it comes to the "posers" who harm themselves for attention. How many of them are truly jumping on the bandwagon, and how many of them could possibly have their own psychological disorder when it comes to acceptance? This girl I speak of needs help. She's obsessive, and her behavior is erratic.
As far as the difference between cutting and piercings/tattoos/etc. goes..
You're right, Aspasia. Society today is highly confusing. How is it that cutters are looked down upon and scorned for their behavior, yet piercings and tattoos are considered stylish and cultural? That is a bit messed up; however, I do believe there's a difference. There are many forms of self-harm, but I'm going to focus on cutting when comparing it to piercing/tattooing. In fact, some may consider piercing and tattooing as other forms of self-injury. My mother freaked when I pierced my lip by myself. She sat me down and asked me, "Is this another form of cutting for you?" Honestly, my answer is yes. Oftentimes, when I'm on the lower point in my emotional cycle, I have a strong urge to pierce something. I obsess over the thought, but I have to restrain myself because piercings are not something I want to hide, like cuts. Therefore, body jewelry is noticable. I'd get caught.
With that said, I truly think there is a difference. Psychologically, not for some people. But cutting presents more of a danger than piercing or tattooing. Tattooing is normally done by a professional (and if you do it yourself and you're not experienced, you're just crazy). On the very rare chance that something lethal goes wrong, sue. Go to the hospital. Piercing is done by a professional, or by an amateur (I pierced my lip twice myself). If you do it yourself, infection can occur. You may hit a vein. This could be a problem, depending on where it is you are piercing. However, piercing your body is still not nearly as dangerous as cutting. Cutting is never done by a professional [with the exception of surgeons (lame joke)]. Often, cuts are deep--they are meant to cause harm. Cutting too deep into the skin, in the wrong direction, etc. can have horrible consequences. Death. Cutting, in extreme cases, is used as a method of suicide: slitting the wrists, the throat. So, although cutting, piercing, and tattooing are all tied together in that they inflict harm/pain on the body, cutting more or less has negative intentions, and much worse consequences.
(Also, a side note to dr_zombie: telling your parents isn't always the solution. What I mean is, one cannot assume that once one tells one's parents about issues with self-harm that everything will go away and all will be just dandy. I thought that telling my parents would save me; it didn't. They cared enough to taxi me to the nearest hospital and get me some help, but that's when they dropped the issue. I've cut many times since then, and they've seen the growing number of scars on my leg. Yet, they never ask. They assume that I'm all happy-go-lucky, and that I don't hurt myself anymore.)