In fact, happiness is one of the most highly heritable aspects of personality. Twin studies generally show that from 50 percent to 80 percent of all the variance among people in their average levels of happiness can be explained by differences in their genes rather than in their life experiences. 28 (Particular episodes of joy or depression, however, must usually be understood by looking at how life events interact with a person’s emotional predisposition.)

Introversion- along with its cousins sensitivity, seriousness, and shyness- is now a second-class personality trait, somewhere between a disappointment and a pathology. Introverts living in the Extrovert Ideal are like women in a man's world, discounted because of a trait that goes to the core of who they are. Extroversion is an enormously appealing personality style, but we've turned it into an oppressive standard to which most of us feel we must conform.

Errors of reductionism and biodeterminism take over in such silly statements as “Intelligence is 60 percent genetic and 40 percent environmental.” A 60 percent (or whatever) “heritability” for intelligence means no such thing. We shall not get this issue straight until we realize that the “interactionism” we all accept does not permit such statements as “Trait x is 29 percent environmental and 71 percent genetic.” When causative factors (more than two, by the way) interact so complexly, and throughout growth, to produce an intricate adult being, we cannot, in principle, parse that being’s behavior into quantitative percentages of remote root causes. The adult being is an emergent entity who must be understood at his own level and in his own totality. The truly salient issues are malleability and flexibility, not fallacious parsing by percentages. A trait may be 90 percent heritable, yet entirely malleable.

Introverts living under the Extroversion Ideal are like women in a man’s world, discounted because of a trait that goes to the core of who they are. Extroversion is an enormously appealing personality style, but we’ve turned it into an oppressive standard to which most of us feel we must conform

We're told that to be great is to be bold, to be happy is to be sociable. We see ourselves as a nation of extroverts — which means that we've lost sight of who we really are. Depending on which study you consult, one-third to one half of Americans are introverts — in other words one out of every two or three people you know.

The adult being is an emergent entity who must be understood at his own level and in his own totality. The truly salient issues are malleability and flexibility, not fallacious parsing by percentages. A trait may be 90 percent heritable, yet entirely malleable. A twenty-dollar pair of eyeglasses from the local pharmacy may fully correct a defect of vision that is 100 percent heritable. A “60 percent” biodeterminist is not a subtle interactionist, but a determinist on the “little bit pregnant” model.

primary mental abilities and perceptual and motor skills are the most influenced by heredity, while personality traits are the least influenced.

Introversion — along with its cousins sensitivity, seriousness, and shyness — is now a second-class personality trait, somewhere between a disappointment and a pathology.

Nor are introverts necessarily shy. Shyness is the fear of social disapproval or humiliation, while introversion is a preference for environments that are not overstimulating. Shyness is inherently painful; introversion is not. One reason that people confuse the two concepts is that they sometimes overlap (though psychologists debate to what degree). Some psychologists map the two tendencies on vertical and horizontal axes, with the introvert-extrovert spectrum on the horizontal axis, and the anxious-stable spectrum on the vertical. With this model, you end up with four quadrants of personality types: calm extroverts, anxious (or impulsive) extroverts, calm introverts, and anxious introverts. In other words, you can be a shy extrovert, like Barbra Streisand, who has a larger-than-life personality and paralyzing stage fright; or a non-shy introvert, like Bill Gates, who by all accounts keeps to himself but is unfazed by the opinions of others.

"Extroverts are more likely to take a quick-and-dirty approach to problem-solving, trading accuracy for speed, making increasing numbers of mistakes as they go, and abandoning ship altogether when the problem seems too difficult or frustrating. Introverts think before they act, digest information thoroughly, stay on task longer, give up less easily, and work more accurately. Introverts and extroverts also direct their attention differently: if you leave them to their own devices, the introverts tend to sit around wondering about things, imagining things, recalling events from their past, and making plans for the future. The extroverts are more likely to focus on what's happening around them. It's as if extroverts are seeing "what is" while their introverted peers are asking "what if.

The source of extraversion or introversion was in the varying levels of excitability of the brain; the driver

It makes sense that so many introverts hide even from themselves. We live with a value system that I call the Extrovert Ideal — the omnipresent belief that the ideal self is gregarious, alpha, and comfortable in the spotlight. The archetypal extrovert prefers action to contemplation, risk-taking to heed-taking, certainty to doubt. He favors quick decisions, even at the risk of being wrong. She works well in teams and socializes in groups. We like to think that we value individuality, but all too often we admire one type of individual — the kind who’s comfortable “putting himself out there.

A preference for extraversion (seeing life in terms of the external world) or introversion (greater interest in the inner world of ideas) is independent of your preferences for sensing, thinking, intuition, and feeling.

Hardly anybody ever writes anything nice about introverts. Extroverts rule. This is rather odd when you realise that about nineteen writers out of twenty are introverts. We are been taught to be ashamed of not being 'outgoing'. But a writer's job is ingoing.