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CBS New York quote author Gretchen Rubin as saying:

“About 50 percent of happiness is genetically determined"

Similarly, Lifecho say:

According to some studies our happiness level depend 50% on our genetics, 10% on circumstances and 40% on our intentional activities.

They refer to Dresdner Kleinwort Wasserstein research to support this.

I am unclear on what this claim means: Does this mean half the time you're happy it's because of genetics, or half the time you're unhappy it's because of genetics, or at any time you are happy 50% of the reason you are happy is because of genetics? How can happiness be measured, in a scientifically reliable way?

Is there evidence to support these claims?

  • Questions that are "I am unclear on what this claim means" are by definition of topic by our standards. This question is about the definition of words and not about empirical claims and thus off-topic. – Christian Apr 27 '16 at 13:11
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    Is citing John Lennon appropriate? – user5341 Apr 27 '16 at 15:57
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Typically, these "50% genetic" claims are misleading simplifications of the longer claim: "50% of the variance in the population can be explained by genetic differences." It is a measure of how accurate a statistical model could be, if it only considered one factor. That appear to be how it is intended here.

The 50% figure seems to have been popularised in a book The How of Happiness by Dr Sonja Yubomirksy, a professor at the University of California.

This conclusion was drawn in her 2005 paper, Pursuing Happiness: The Architecture of Sustainable Change (with co-authors, Kennon M. Sheldon and David Schkade):

First is the idea of a genetically determined set point (or set range) for happiness. Lykken and Tellegen (1996) have provided evidence, based on twin studies and adoption studies, that the heritability of well-being may be as high as 80% (although a more widely accepted figure is 50%; Braungart, Plomin, DeFries, & Fulker, 1992; Tellegen et al., 1988; cf. Diener et al., 1999). Whatever the exact coefficient, its large magnitude suggests that for each person there is indeed a chronic or characteristic level of happiness

The OP expressed skepticism about the quality of Happiness research, so let's look at those references:

  • Genetic influence on tester-rated infant temperament as assessed by Bayley's Infant Behavior Record: Nonadoptive and adoptive siblings and twins. By Braungart, Julia M.; Plomin, Robert; DeFries, J. C.; Fulker, David W. Developmental Psychology, Vol 28(1), Jan 1992, 40-47.

    This looked at the temperament (along three factors: affect-extraversion, activity, and task orientation) for infant twins. "Both the sibling adoption and twin data yielded evidence for significant genetic influence for the 3 IBR factors at 12 and 24 mo: Heritability estimates ranged from 35–57%."

    So, this didn't measure adults and didn't measure "happiness" but external temperament, but suggests that personality is heritable.

  • Tellegen, A., Lykken, D. T., Bouchard, T. J., Wilcox, K. J., Segal, N. L., & Rich, S. (1988). Personality similarity in twins reared apart and together. Journal of Personality and Social Psychology, 54, 1031–1039.

    A personality questionnaire, MPQ was administered to over 300 pairs of adult twins.

    Some of the factors that were measured include "Well-Being" and "Positive Emotionality".

    The variance of "Well-Being" was 48±8% accounted for by genetics. The variance of "Positive Emotionality" was 40±8% accounted for by genetics. [See Table 5]

  • Lykken and Tellegen (1996), Happiness is a stochastic phenomenon:

    This paper builds on the results of the previous paper.

    Happiness, or subjective well-being, was measured on a birth-record-based sample of several thousand middle-aged twins using the Well-Being (WB) scale of the Multidimensional Personality Questionnaire. Neither socioeconomic status, educational attainment, family income, marital status, nor an indicant of religious commitment could account for more than about 3% of the variance in WB. From 44Vc to 52% of the variance in WB, however, is associated with genetic variation. Based on the retest of smaller samples of twins after intervals of 4,5 and 10 years, we estimate that the heritability of the stable component of subjective well-being approaches 80%.

    This 80% figure seems to be partly speculative estimate based on the expected long-term retest stability of the questionnaire. (That is, how much someone will give the same sort of answers to the questionnaire ten years later.)

  • Diener, E., Suh, E. M., Lucas, R. E., & Smith, H. L. (1999). Subjective well-being: Three decades of progress. Psychological Bulletin, 125, 276–302.

    This is a review paper, and examines many of the results above, and compares them to other results, suggesting caution with the 80% figure quoted above.

    I can't see anywhere in the paper where they draw a firm figure - the paper is more qualitative, than quantitative - but I would suggest if you wanted to understand the general state of play of the research (as of 1999), this would be a good paper to read.

Summary

There are several studies that measure factors of human emotional temperament and well-being that could reasonably described as "happiness" in an informal setting. Genetics account for a large part of the variability (i.e. you can make a good guess about someone's temperament if you subject their identical twin to a questionnaire), but the exact percentage of the variability accounted for varies between studies techniques. A 50% estimate is a reasonable short-hand.

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    Well that's a depressing thought if 50% of what determines a person's happiness is out of their control. – Celeritas Apr 27 '16 at 4:42
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    @Celeritas There may be more than genetics influencing you as well. Prenatal development can also affect your temperament in later life: books.google.com/… – called2voyage Apr 27 '16 at 14:05
  • Would it be a misinterpretation of the statement to say "a person can vary their happiness level by at most 50%"? Just because 50% of the factors that determine the level are genetic, doesn't mean the level can't be varied and says nothing about the range of the level. Though I may be overthinking this. – Celeritas Apr 30 '16 at 0:09
  • While it may be true 50% of the happiness level is determined by genetics, the statement doesn't say anything about the range or variability of the happiness level.For example I have a happiness level of 100. I can change to a happiness level of 200 as it still could be in the range. Or does the statement mean the most would be level 150? – Celeritas Apr 30 '16 at 0:13
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    Yes, that would be a misinterpretation. You are right that it says nothing about the levels nor whether they can be varied in individuals. – Oddthinking Apr 30 '16 at 0:15

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