Have a novel, with two characters. Who have a hereditary mental disorder, their father is mentioned to have suffered in the same way. Now it would be easy to make them schizophrenic, considering hallucinations and at least one of them has an odd way of speaking. However, that's just to simple and I like to make things complicated because I said so.
I know. I'm a walking schizophrenic. Maybe sitting is the right word. I just found schizophrenia to be typical. When a writer wants to be shocking, blame it on the schizophrenia.
Ok, let me say this Schizophrenia is kinda the only Mental Disorder proved to have an hereditary factor. Sure there are other things like Alzheimer or Parkinson, but those are more like neurological diseases than mental disorders. Now, what's key to develop Mental disorders? asides from a certain inmunology configuration, environment, shocking stuff, specially on young age. Also for adults shocking stuff works also, in a more different way. But hey!, do you want to be a complete bastard with your character? give 'im some wrecked childhood. Also you could add some personality issue, this is often tied with the chances of developing a mental disorder xD. The problem?... although it can happen and is not unfrequent to have all these factors.... it would be a bit too much for a character in a book, what do I mean?... it could feel a bit unreal, like, all bad stuff casually hitting these two guys, ya get my drift?
It's a novel about a family of serial killers. I don't think wrecked childhood is even the beginning of it.
I approve this xD. But returning to the topic. What do you want is 1) An idea to make their thing more complicated? 2) Another mental disease for their already fucked mind?
A mental disease for their already fucked mind. Cause there is nothing more disturbing on how innocent this family takes murdering people.
Personality disorders are classified in CLUSTERS Cluster A: Weird, Introvert, Shut-in kind of people (I don't mean the common introvert, something extreme). This is the most fucked, schizophrenia is here. Cluster B: Emotional and dramatic. Not drama queens.... super drama queens xD. Cluster C: Anxious and fearful.
Now, which one is really fucked and moreover in that kind of family? Borderline personality, Cluster B. Seek for that one.
Uhhh. Excuse me. But Schizophrenia is certainly NOT the only mental disorder that is heriditary. I challenge any one who thinks so. My father was once diagnosed with clinical depression. I'm BiPolar (aka manic-depressive). To compound that problem, I have a bad case of PTSD. I had 4-5 years of dangerous amounts of stress during a very trying and traumatic situation which compounded my problems with BiPolarism 100 fold. (And depression is often linked with murders and suicides. Not that either my father or I are that bad off.) I'm also ADD (undiagnosed, just know I am because of a profound issue of PAYING ATTENTION) and can be easily distracted. You don't have to have a "complicated" (what exactly does that mean any way? they can all be complicated) disorder. You can have it "simple", like depression, and compound the problem by adding to it. A Dr of mine once said that mental disorders don't usually sit on their own. They are often accompanied by other disorders that can certainly compound/complicate their other disorders. Ok. I've said my two cents worth.
If you want the facts, check out this article from the NIMH http://www.nimh.nih.gov/news/science-news/2013/five-major-mental-disorders-share-genetic-roots.shtml
As a psychiatrist, it really pains me to see these concepts thrown around so inelegantly, as if mentally ill people are concepts and tropes to be employed, as if mental illnesses are some kind of entities thats should be enough to explain the most depraved behaviour. Reading the criteria in DSM can certainly give a strong impression, but that is not a reality or real people, nor is it all that affects a person. So when a writer says 'give them borderline! give them schizophrenia!' (to explain serial killing family) it is such a deeply flawed and ignorant character building, it should be strongly discouraged, mostly because it results in ludicrous premises, but also because it deepens prejudice against mentally ill people, who as a group have a a very low propensity for violence, much lower than say young men as a group. A lot of mental illnesses are hereditary to a degree, bipolar is the big one, but also any mood and anxiety disorder, OCD, schizophrenia etc. There is no 'one gene equals illness' but children of mentally ill parents have a higher chance of developing that illness than general population. Psychopathy also can run in families, there have been various pieces of research correlating certain findings on brain scans with potential for violence, but more than anything, it was the early experience of sadistic parenting, narcissistic personality and personal proneness to violence, that resulted in seriously disturbed individuals. It is possible that the entire family is involved in kidnapping and serial murder, this has happened before and is a well-known phenomenon in criminology and forensic psychiatry. In such instances, you often have the killer couple (have a look at the cases of Myra Hindley and Ian Brady or Rose and Fred West) where usually one is a dominant sadist, most often a man who is controlling his wife and making her into an accomplice, but sometimes the woman is the primary manipulator who is getting a man to do what she requests. And they might have narcissistic or borderline or sadistic or dependant personalities (which are personality disorders, not frank mental illness), but so do millions of other people who never turn violent. Should such people have children, they would be born into this depravity, and would most likely participate in murders because they would be conditioned to do so since birth. Often in such cases killer's children are used to lure in the new victims for example, and the rite of passage is to commit murder themselves. All this comes at immeasurable psychological expense to those kids. Serial killers are rarely 'mentally ill', as in depressed, manic or psychotic, but if they are, they tend to be disorganised and easily caught. It is important to research these issues before reaching for 'crazy peoples are killing in a gang' trope, which is completely inconsistent with what actually happens in such situations. Research real cases of serial killers, you'll find a wealth of authentic information of what kind of people do these horrible crimes.
To the OP- @jazzabel pretty much summed it up, a mental disorder is not a quick slap ticket to violence and murder. Mentally ill people are hardly always violent, and that includes the popular cards of schizophrenia and OCD. What's simple is using a mental illness as lieu for criminal behavior--while criminal behavior in itself may suggest the person does not think ordinarily or as expected, it doesn't mean they are completely out of it. It's simple, it's cliched and it's not entirely factual. A point to be noted is, while you may think many mental illnesses are not heritable (when in fact, as pointed out, many are), what can run in the family is dysfunctional patterns of behavior. Systems theory of psychotherapy emphasizes that a disordered individual, in any sense of the word, is not a product of one-way traffic of bad genes or bad influence. Dysfunctional individuals signify a dysfunctional system, so much so that even if you help the individual, he/she will revert to bad habits because of these patterns of interaction. Perhaps the problem is not the gene X, but the family's culture, inherent beliefs, unspoken rules of interaction, and what. Kind of like the social norm, except on a micro level. This in itself may not sound particular novel, since we write about dysfunctional families all the time, but perhaps it will help you explore better the kinds of character dynamics you can use to be both authentic and original. Think, what kinds of people become part of cults? What kinds lead cults? What kind of a woman would stick by an abusive husband--what is wrong with her? In parts of the world, woman encourage their husbands to rape the maidservants in front of them. What is their mindset? What kind of system do they identify with. To me, that would be much more revealing that a diagnosis from DSM.
@jazzabel gave a good account of why 'mental illness' may not be a good way to approach this kind of story. It implies that mentally ill people are killers, which is not any more representative than saying 'people are killers.' ...and then @Charisma discussed what I was thinking myself. The influence of environment. When it comes to the split between nature and nurture in determining behaviour, I'll nearly always go for nurture when I'm writing about it. The way a person is 'brought up' can lead to all sorts of story ideas. A bad beginning can certainly lead to a bad end ...but so many times real people 'rise above' a bad beginning. This is another avenue that makes a good story. A really important thing to consider when deciding on this issue is this: will your character be responsible for their behaviour or not? A person who is genuinely mentally ill exists in a different reality from the rest of us. I don't feel they can be held totally responsible for their actions. (The law generally supports this view.) A criminal who does not suffer from mental illness but comes from a bad background instead, may be deserving of sympathy and understanding, but they can't be totally let off the hook. At the worst their bad behaviour may be incorrigible, but at best they may flourish if their environment is changed for the better. All they need is a vision of what they can change, and an opportunity to do so. They will either pursue that vision or not, but it's their choice. A mentally ill person doesn't really have that choice (although it's all dependent upon degree, I suppose.) So ...do you want your POV character to be essentially blameless or not? As a writer, I reckon that's the question you need to answer.
The problem is that you have hallucinations. This is always schizophrenic in nature. However, schizophrenia is rarely violent. Combined with a bipolar factor you may gets ome violence. Then you are working with schizo-affective disorder, which is not always accepted by the American Association of Psychiatry. It is, however, hereditary.
You can also do bipolar as well because people with bipolar can have hallucinations and it is hereditary.