Genes and Personality

Hi GDR, First off, we want to decide whether personalities are inherited at all. The evidence for the inheritance of personality (by which I mean interests, patterns of behaviour, inclinations, etc) is very strong. Child abuse, for example, appears to be strongly heritable. I've lost count of the number of times I've read in a newspaper that some child abuser was abused himself when young. This is why social workers talk about the "cycle of abuse". The cycle of abuse is a cycle of inheritance, and the cycle of behaviour is broken by preventing inheritance. On a more trivial level, we find (in the UK) that many people living in a small town will support the same local football team, so support for that team is clearly inherited in some way. One doesn't wake up one morning and pick a team at random.But this kind of inheritance doesn't follow the same path as genetic inheritance. Genetic inheritance spreads from parents to children (i.e. vertically), whereas cultural "inheritance" can also spread horizontally (i.e. between unrelated members of a population). Here is a (perhaps exaggerated) example. I'm pretty sure those children didn't inherit the behaviour from their parents, but they inherited it culturally by horizontal transmission. Furthermore, unlike inheritance of a genetically determined trait, their inheritance contained at least some volition. To be a happy slapper or not to be a happy slapper, that is the question. It's worth making a digression here to discuss the notion of a physical pathway in cultural inheritance. The personality traits I've mentioned so far are inherited culturally, but are necessarily inherited by a physical pathway. One becomes an abusive parent by being physically abused as a child; one becomes a keen football fan by physically watching and playing games with one's father; etc. These personality traits are also manifested physically: A person who has a love of literature can be identified by examining the books they have purchased, or by measuring the amount of time they spend reading. One can identify more ethereal traits, such as a suicidal tendency, by counting scars on a forearm.For me, there is no dichotomy between cultural inheritance (or "memes" - that ugly, silly and damaging word) and genetic inheritance. They are just different physical routes by which a personality trait moves from person to person. There is nothing metaphysical or spiritual involved, unless one's notion of spirit includes physical conditioning and learned experience. It seems clear that personality traits are physically inherited in some manner, either vertically or horizontally. Some traits (such as a foot fetish, or whatever) may appear to be invented de novo in an individual, without direct inheritance (a mutation event ). The big question is whether personality traits can follow the same physical pathway as the genome.One problem is that, as a previous poster pointed out, the classical method for carrying out genetic inheritance research is to look at mutants carrying a single rare allele with strong effect. Consequently we know much more about genetic disorders than about the normal functioning of the genome. Looking at personality traits exacerbates this problem - because personality traits are exactly the kind of traits that we would expect to be influenced by a large number of genes of small effect, and the experimental protocol is further complicated by the fact that human beings can learn and modify their personalities.Evidence for the inheritance of personality comes largely from twin adoption studies. The idea is that twins are genetically identical, but have been adopted into different families with different patterns of cultural inheritance. If we look at enough twins, and enough diverse family backgrounds, we might be able to identify personality traits that appear to be "immune" to the cultural background. In principle, these studies can be carried out on adopted non-twins, as long as we take their reduced genetic relatedness into account.These kinds of studies have identified some personality traits (agression, propensity to suffer from stress, lethargy, propensity to watch TV, etc) that appear to be inherited. But for reasons outlined above, the level of inheritance is low. Personality traits, because they are likely dependent on an unknown but large number of genes, and because they are susceptible to cultural modification, and because they are very difficult to define and measure consistently, show low penetrance. In the older literature you will often see silly, unscientific and essentially meaningless statements like "27% of aggressive behaviour is genetically inherited". One should be very wary of such statements. What is called a "gene for aggression" when it is expressed on a housing estate in the east end of london, might be called a "gene for being a formidable stock trader" when it is discovered in a sample from Harvard graduate students, or a "gene for nobility" when it is discovered amongst the Yanomamo people. One also has to ponder what 27% of one's aggression means. I'll end with two examples of research from different empirical aproaches that suggests you are wrong.1. Suicide.As expected, the first result is from a "disorder", in this case propensity to commit suicide. Note that the authors have identified a single nucleotide polymorphism (!)Here's the reference. Here's the abstract:

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Genetic factors contribute to the risk of psychopathology in many psychiatric conditions, but the specific genes are yet to be identified. Neurotransmitter alterations are implicated in the etiology of psychopathology based, in part, on studies of neurotransmitter receptors and their biosynthetic or degradative enzymes in postmortem tissue. Identification of the altered receptors and enzymes serves to identify candidate genes of potential etiological significance. Polymorphisms in these genes can contribute to alterations in protein function in vivo that are part of the neurochemical underpinnings of psychopathologies such as major depressive disorder, psychoses, alcoholism, personality disorders, aggressive-impulsive traits, or suicidal behavior. Altered serotonergic function is implicated in the etiology and pathogenesis of several major psychiatric conditions. In particular, there is much evidence for an association of lower serotonergic function and suicidal behavior. Thus genes related to the serotonergic system are candidate genes worthy of study as part of the genetic diathesis for suicidal behavior. This review examines the following polymorphisms in the serotonin biosynthetic enzyme tryptophan hydroxylase (TPH; A779C substitution), the serotonin transporter (5-HTT, 5-HTTLPR allele), the 5-HT1B receptor (G861C, C129T substitution) and the 5-HT2A receptor (T102C) for their relationship to suicidal behavior. For the TPH gene, we found the less common U or A allele variant of the A779C polymorphism was associated with suicide attempt. Other studies have found the U allele to be associated with aggression and lower serotonergic function in vivo. A 44 base pair insertion/deletion in the 5' flanking promoter region of the 5-HTT gene may result in less 5-HTT expression and 5-HTT binding. We examined 220 cases postmortem and found no association between the promoter genotype and 5-HTT binding. We also found no association with major depressive disorder (MDD), suicide or pathological aggression, despite finding significantly fewer 5-HTT sites in the prefrontal cortex of depressed and/or suicide cases. In genomic DNA samples from 178 unrelated subjects, we detected two polymorphisms for the 5-HT1B receptor at nucleotides 861 and 129. However, no association between either polymorphism and depression, suicide, aggression, or alcoholism was observed. There are two common polymorphisms for the 5-HT2A receptor gene in humans. The results of studies of 5-HT2A receptor gene polymorphisms do not indicate significant major associations with suicidal behavior. In contrast, the 5-HT2A receptor itself is reported to be increased in suicide. Functional polymorphisms involving the promoter region that affect gene expression may explain this finding. Studies of candidate genes related to serotonergic function in brain are increasingly used to establish genetic alterations contributing to psychiatric illness. The most meaningful studies combine the study of candidate genes with direct measures of related proteins as well as psychopathology. (C) 2003 Elsevier Ltd. All rights reserved.

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2. HomosexualityNot a disorder, of course. The results are suggestive but due to the type of analysis we're not able to identify a specific nucleotide change this time Here's the reference. Here's the abstract:

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The Darwinian paradox of male homosexuality in humans is examined, i.e. if male homosexuality has a genetic component and homosexuals reproduce less than heterosexuals, then why is this trait maintained in the population? In a sample of 98 homosexual and 100 heterosexual men and their relatives (a total of over 4600 individuals), we found that female maternal relatives of homosexuals have higher fecundity than female maternal relatives of heterosexuals and that this difference is not found in female paternal relatives. The study confirms previous reports, in particular that homosexuals have more maternal than paternal male homosexual relatives, that homosexual males are more often later-born than first-born and that they have more older brothers than older sisters. We discuss the findings and their implications for current research on male homosexuality.

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Hope this is of interest. Sorry for wandering.Mick[edited to correct subtitle]This message has been edited by mick, 07-06-2005 01:29 PM