One way that some of the so-called "biological" psychiatrists twist the truth in order to justify their beliefs that these behavioral problems are due to brain malfunctioning is to try to make the case that they have their origins mostly in genetics. Never mind that there is barely a legitimate neuroscientist alive who believes that any gene or any group of genes specifically codes for any complex human behavior pattern. 

Most of the "experts" who are now in the process of misleading the field by exaggerating genetic influences do concede that both genetic and environmental factors play a part in creating behavioral syndromes. They have to, since the rate at which genetically identical twins both show the syndromes is almost never anywhere close to 100%. It is usually quite a bit below that.  If some behavior were solely determined by genetics, and if one identical twin shows it, the other would have to as well.  

What they have done is come up with a way to "apportion" supposed causative factors into genetic and environmental ones using a statistic called heritability. 

Using studies of identical twins who were raised together versus those who were raised apart, they purport to estimate the variance, or how much each of the various causative factors contributes to a certain disorder. The variance is expressed as a percentage of the total package. Variances are thusly assigned to genetic factors, "shared" environmental factors such as growing up in the same household, and "unshared" environmental factors for twins raised apart.

In actuality, a determination of which parts of an environment are shared by siblings and which are unshared has a lot in common with finding water with a divining rod. Anyone foolish enough to think that parents treat all of their children in anything remotely close to an identical manner must have no siblings and no more than one child. But more on that shortly.

The misleading tactic is to use the twin study statistic called heritability as a synonym for genetic. It is not. The statistic derived from twin studies is not a measure of genotype but of phenotype. Genotype refers to the actual sequence of molecule pairs in the DNA of which an individual's genes are made. Phenotype, on the other hand, is the final result of the interaction between genes and the environment - in other words, what you actually see in the whole organism and its behavior. 

A key point to remember is that almost all of the genes in any given cell, even the ones that are at times active in a given type of cell such as a neuron, are in the "off position" most of the time. What turns them on or off are environmental influences! In regards to behavioral issues, the social environment is especially important. One of the main purposes of the brain is, in fact, to interact with other brains.

The heritability statistic is actually a mix of purely genetic influences plus gene-environment interactional influences. There is no way to tell how much of each is present in the statistic. The determination of heritability can also be manipulated in a number of ways, such as by setting the bar for saying that a syndrome is present or absent. How much and how often does one have to drink to be called an alcoholic, after all?

All human behavior, normal or abnormal, has a genetic component. That's because genes determine what the brain is capable of or incapable of, what it has a tendency to do and a tendency not to do, etc. Our genes provide a range of options. They do not specify what behavior within that range will occur in a given environmental context. To say that genes play a role in creating a behavioral syndrome is a tautology—a rose is a rose.

The absurdity of using heritability as a synonym for genetic is beautifully illustrated by a study published in the Journal of Adolescent Health (V.45 [6]:579-86, Dec. 2009) by van der Aa et. al. They looked at the heritability of high school truancy. The study pegged the "genetic" influence on this behavior at 45%!! Does anyone seriously believe that ditching school is determined to a significant degree by heredity?

Going back to the matter of determining which parts of the environment of two siblings being raised in the same or different households are "shared" and which are "unshared": Just because two children are close in age and grew up in the same household, this does not mean that they had even remotely similar experiences.             

Unbelievably, I still occasionally hear the argument that a particular behavioral disorder could not possibly be shaped primarily by dysfunctional relationships with parents, because siblings of the offending parents have turned out completely differently. That siblings turn out differently is quite true. In fact, they can and often do turn out to be polar opposites! In some families, for example, one son becomes a workaholic and the other a lazy freeloader who refuses to keep a job. I have difficulty imagining a shared genetic mechanism that would lead to an outcome like that, but it can be easily explained by looking at family dynamics and psychology.

The Smothers Brothers comedy duo made an entire career out of feigned sibling rivalry summed up by Tommy Smother's catch phrase, "Ma always liked you best." Clearly this theme resonated with a lot of people. Does anybody really treat all of their children in a nearly identical manner? How could they? Children are born with major differences from one another that force parents to react differently to them even if the parents try not to. Even more important, anyone who thinks that some parents do not pick out some of their children to treat like Cinderellas and others to treat like princesses has his or her head in the sand. Or some other dark location that I'm too polite to mention.

In some ethnic groups, contrasting and seemingly unfair treatment of siblings because of their birth order is actually mandated by the culture. For example, in some Chinese families the oldest son often is groomed to inherit the family business, while a younger brother inherits much less if anything. In many Mexican American families, the oldest daughter has the duty to look after her younger siblings. She may have to forego her own high school social life in order to do so, while her younger sister has far fewer family obligations and gets to party on. Of course, parental behavior is not the only influence on how children turn out after they grow up, but it remains one of the most important ones.