Behavioural genetics refers to genetic research on behavioural differences between people, just medical genetics refers to genetic research on medical disorders. Indeed, there’s nothing special about genetic research on behaviour – the same genetic research methods can be applied to any trait – physical, physiological or psychological.

Surprisingly, there is hardly any research on this topic. We know that people who are generally anxious are more anxious about most things including strange dogs (and people). However, in my experience there is something specific about fear of dogs. When I walk my dog in the park and come across a parent and young child, I can predict which kids are going to want to play with my dog and which are going to freak out. It’s in the parent’s reaction –whether they’re drawing away looking anxious or smiling. So, contrary to what my research has shown me that such differences in parenting don’t have much of an effect on their children, this is one case where I’m willing to be there are systematic influences of the parent. It’s an empirical issue.

This is a common misunderstanding of genetics, which involves propensity not destiny. I have a high polygenic score for obesity, which some people are surprised to learn is highly heritable. This doesn’t mean I am destined to become fat. If I ate less and exercised more, I wouldn’t be fat. However, for you skinnies out there, you need to know that it is much easier for genetic fatties like me to put on the pounds and much harder to lose them.

As in my response to several other questions, all psychological dimensions and disorders show significant genetic influence. Genetic research on personality disorders is summarised in the 7th edition of our textbook, Behavioral Genetics (pp. 266-267).

As mentioned in another response, I have a section in Blueprint called ‘Parents matter, but they don’t make a difference’ (pp. 82-86). Parents obviously matter tremendously in their children’s lives. But differences in parenting – the affection, discipline, control they provide – do not make a difference in how their children turn out in the long run. Environmental differences explain the other half of the differences between children not explained by genetics, but these environmental influences are not the systematic effect of differences in parenting. My point is that you would be much the same person you are now if you had been switched in the hospital nursery and raised in a different family with different parents. This isn’t just a thought experiment – a dramatic illustration of this can be seen in an award-winning 2018 documentary called Three Identical Strangers. (These issues are described at length in my book, Blueprint: How DNA Makes Us Who We Are, and is the topic of the next book I am writing with the working title, Parenting after the DNA Revolution.)

Environmental theories from Freud onwards assumed that the family was the mechanism by which environmental influences were doled out. This is was is meant by nurture. Such theories have a tough timing explaining why children growing up in the same family like you and your three sisters are so different. However, genetics has no problem explaining this. Siblings are 50 percent similar genetically, but that means they are 50 percent different genetically. So why are siblings in the same family so different? The answer is genetics and non-shared environment. Growing up in the same family does not make siblings similar environmentally. In other words, you and your sisters would have been just as similar if you had been adopted at birth and all raised in different families with different parents. I wrote a book with Judy Dunn called, Separate Lives: Why Siblings Are So Different.

In order to create polygenic scores, we need genome-wide association studies that genotype hundreds of thousands of DNA differences for hundreds of thousands of people. We need so many DNA differences because we need to look for the many DNA differences that affect common disorders and complex traits across the three billion steps in the spiral staircase, double helix, of DNA. We need so many people in these studies because we are trying to find many DNA differences each of which have tiny effects. My team and I did the first genome-wide association study of psychopathic tendencies, but the study was too small to find anything. There have been subsequent studies but they are still not large enough to have much success. It will happen, however, because psychopathic tendencies, especially the callous-unemotional core of psychopathy, is highly heritable. (This is described at length in my book, Blueprint: How DNA Makes Us Who We Are.)

As mentioned in several responses, all psychological traits show significant genetic influence. Personality traits are about 40 percent heritable and cognitive traits are about 60 percent heritable. Psychopathology has a range of results, from less heritable traits like depression to more heritable traits like bipolar-manic depression.

Yes, polygenic scores are exactly what I’m talking about. They are derived from (This is described at length in my book, Blueprint: How DNA Makes Us Who We Are.) genome-wide association studies

Even though DNA testing began just a few years ago, it is already more powerful than other predictors of behavioural traits. The strongest DNA predictor is for school performance. We can now predict 15 percent of the differences (variance) between children in their performance on national exams in the UK at the age of 16. Those test scores are about 60 percent heritable which means we have a lot of head room to discover more DNA differences that predict school performance. However, for comparison, how much variance in school performance do you think can be predicted by differences in school quality. The answer in a paper my group is just about to publish is 4 percent. So, DNA is more than three times stronger in predicting school performance than school quality. These DNA predictors will continue to get stronger. (This is described at length in my book, Blueprint: How DNA Makes Us Who We Are.)

DNA testing can be done through direct-to-consumer companies such as 23andMe for about $100.

I assume you are asking if genetics can influence traits like life satisfaction. If so, the answer is ‘yes’. As mentioned in response to other questions, an amazing finding from behavioural genetics is that all psychological traits show significant genetic influence.

One of the most surprising findings from behavioural genetics is that all psychological traits are influenced substantially by inherited DNA differences, none of these traits are ‘derived solely from our genetics’. Heritability, which is an index of the strength of genetic influence from 0 to 100 percent, is about 50 percent on average for psychological traits. Heritability is about 40 percent for personality traits and about 60 percent for cognitive traits.

This is one of the most important findings from behavioural genetics. When I say that inherited DNA differences are the major systematic force making us who we are as individuals, accounting for about half of the differences between people. The other half is due to the environment but these environmental influence are not the systematic effects connoted by the word ‘nurture’, such as parenting and the family environment. After three decades hunting for these non-shared environmental influences, I have come to conclude that, although important, these environmental influences are unsystematic and idiosyncratic – in other words, chance. We do not have control over these chance factors. (This is described at length in my book, Blueprint: How DNA Makes Us Who We Are.)

As mentioned in my responses to an earlier question, heritability does not imply immutability. That is, genetic influence refers to propensity, not destiny. For example, if you knew you were at high risk for alcoholism, this does not mean that you are destined to become an alcoholic. The heritability of alcoholism means that if you drink as much alcohol as other people, you would be at greater risk for becoming an alcoholic than them. However, you cannot become an alcoholic unless you drink a lot of alcohol over a long period of time.

Whenever selection is involved – for example, in education and occupations – I believe that DNA testing will eventually be used to supplement other selection criteria. In Blueprint, I mentioned that DNA testing would be extended to selecting partners on dating websites – this is now one of the hottest areas for DNA testing companies, DNA dating!

Although inherited DNA differences influence all common disorders and complex traits, it is not appropriate to talk about ‘a gene for’ any of these. They are influenced by many genes of small effect, unlike single-gene disorders that are rare. But if you are asking if there is genetic influence on traits like faithfulness and infidelity, then the answer is yes. All personality traits show some moderate genetic influence.

Many researchers have looked at the heritability of autistic traits, which seems to be among the most heritable of childhood behaviour problems. Our research suggested that isn’t one ‘thing’ genetically – each of the three symptoms (theory of mind, communication, repetitive behaviour) -- are influenced by different genes.

My former PhD student, now Professor Essi Viding, first studied the callous-unemotional aspect of psychopathy and found it much more heritable than other aspects of antisocial behavior, conduct disorder and ODD. Just because such traits are heritable does not mean that you can’t do anything about them. (This is described at length in my book, Blueprint: How DNA Makes Us Who We Are.)

A phenotype is a trait that we assess, whether it is physical (e.g., eye color, height, weight) or psychological (e.g., personality or psychopathology).

For the thousands of single-gene disorders, we can talk about a gene causing the disorder in that the genetic mutation is necessary and sufficient for the appearance of the disorder. However, we talk about ‘influence’ rather than ‘cause’ when we consider complex traits like body mass index and common disorders like obesity because these are caused by a single gene but rather by thousands of inherited DNA differences of small effect. (This is described at length in my book, Blueprint: How DNA Makes Us Who We Are.)

In Blueprint, I have a section called ‘Parents matter, but they don’t make a difference’ (pp. 82-86). Parents obviously matter tremendously in their children’s lives. But differences in parenting – the affection, discipline, control they provide – do not make a difference in how their children turn out in the long run. Environmental differences explain the other half of the differences between children not explained by genetics, but these environmental influences are not the systematic effect of differences in parenting. (I am currently writing a follow-up to Blueprint which will focus on genetics and parenting – it’s working title is Parenting after the DNA Revolution.)

One of my mantras is that heritability does not imply immutability. Heritability means that on average in the populations studied in genetic research, inherited DNA differences account for differences in neuroticism. That is, genetic research describes ‘what is’ – it doesn’t limit ‘what can be’. (This is described at length in my book, Blueprint: How DNA Makes Us Who We Are.)

You can’t find a single psychological trait that can be measured reliably that does NOT show genetic influence. So, no surprise, individual differences in smoking and nicotine show moderate genetic influence. There are several interesting issues here, which are discussed on pages 284-288 of the 7th edition of our textbook, Behavioral Genetics.

I think the more we know about ancestry, the more we realise what a hodge-podge of ancestral DNA we all are. At the end of the paperback edition of my book Blueprint, in a section called ‘Afterword’ (pp. 193-194), I talk about this issue of eugenics as a question I am frequently asked. Here’s one aspect of the conversation that I find interesting. Totalitarian regimes like Nazi Germany do evil things, but they don’t need a rationale to do it. The Nazis misappropriated genetics as a rationale to justify their atrocities. However, most totalitarian regimes assume an environmental rationale: that people can be moulded as the state wishes. For example, Stalin’s USSR, the 1948 model for George Orwell’s 1984, actively denied genetics and purged scientists who believed in it. During Stalin’s Great Purge, millions of citizens deemed ‘enemies of the working class’ were imprisoned or killed. Other examples of totalitarian regimes with an environmental rationale include Mao’s China and the Kim dynasty in North Korea.

Height and eye color are very highly heritable, IQ is moderately heritable, and personality is modestly heritable. So, yes, your choice of a donor will affect these traits in your child.

I wish more people recognized the strong genetic contribution to weight to step back from the blaming and shaming that we see today. I don’t see why DNA testing would lead to the marginalization of individuals with obesity. My polygenic score for body mass index is my highest DNA risk score – my polygenic score is at the 94th percentile. It has been helpful for me to know that. (This is described at length in my book, Blueprint: How DNA Makes Us Who We Are.)

My view is that the power of labelling is greatly overrated. It would be terrific if all we had to do is tell children they’re great and can be anything they want to be if they just try hard enough. This labelling is not harmless – it implies that if you fail it’s your fault in that you didn’t try hard enough. Parents and teachers’ perceptions of a student may be preconceived as you say but I think these perceptions are based on children’s performance. So, I don’t think that DNA testing will have major effects due to labelling.

The neat thing about inherited DNA sequence is nothing changes it from the moment you began life as a single cell. All the other trillions of cells in your body have the same DNA, which means that a baby’s DNA at birth can predict their traits as an adult.

You might be thinking about gene expression, which doesn’t change inherited DNA sequence. (This is distinction is described at length in my book, Blueprint: How DNA Makes Us Who We Are.)

After my 50 years of research on this topic, I have concluded that inherited DNA differences are the major systematic force making us who we are as individuals – our personality, our mental health and illness and our mental abilities and disabilities. (This is what my book is about: Blueprint: How DNA Makes Us Who We Are. If you meant, what are the biological mechanisms by which genes influence behaviour, that’s also described in Blueprint.)

If you are referring to DNA genotyping as in 23andMe, DNA testing can tell you with certainty about the thousands of single-gene disorders. For common disorders like heart attacks and psychological dimensions, thousands of inherited DNA differences contribute to differences between people when we aggregate these DNA differences into what is called a polygenic score. You can make a pretty good prediction of a newborn’s height from their DNA polygenic score. The strongest prediction in the behavioural sciences is a polygenic score for school performance – it accounts for about 15 percent of the differences between children in the UK at age 16 on nationwide tests of school performance. (This is described at length in my book, Blueprint: How DNA Makes Us Who We Are.)

After my 50 years of research on this topic, I have concluded that inherited DNA differences are the major systematic force making us who we are as individuals – our personality, our mental health and illness and our mental abilities and disabilities. (This is what my book is about: Blueprint: How DNA Makes Us Who We Are. If you meant, what are the biological mechanisms by which genes influence behaviour, that’s also described in Blueprint.)