#### What does heritability mean?

Individuen unterscheiden sich in einer Vielzahl von Merkmalen voneinander: Bekannte Beispiele sind Größe, Intelligenz und Aspekte der Persönlichkeit. Diese Unterschiede sind oft von erheblicher sozialer Bedeutung. Viele interessante Fragen können über ihre Natur und Herkunft gestellt werden. Eine solche Frage ist, inwieweit sie Unterschiede zwischen den Genen der beteiligten Personen widerspiegeln, im Unterschied zu Unterschieden zwischen den Umgebungen, denen diese Personen ausgesetzt waren. Hier geht es nicht darum, ob Gene und Umgebungen für die Entwicklung eines bestimmten Merkmals wesentlich sind (dies ist immer der Fall), und es geht nicht um die Gene oder die Umgebung einer bestimmten Person. Wir befassen uns nur mit der beobachteten Variation des Merkmals zwischen Individuen in einer bestimmten Population. Eine Zahl, die als "Erblichkeit" (h2) des Merkmals bezeichnet wird, repräsentiert den Anteil dieser Variation, der mit genetischen Unterschieden zwischen den Individuen verbunden ist. **Die verbleibende Variation (1 - h2) ist mit Umgebungsunterschieden und Messfehlern verbunden.** Diese Anteile können mit verschiedenen Methoden geschätzt werden

**For the complex traits that interest behavioral scientists, it is possible to ask not only whether genetic influences are important but also how much genetics contributes to the trait.** The question about whether genetic influences are important involves statistical significance, the reliability of the effect. For example, we can ask whether the resemblance between “genetic” parents and their adopted offspring is significant or whether identical twins are significantly more similar than fraternal twins. Statistical significance depends on the size of the effect and the size of the sample. For example, a “genetic” parent-offspring correlation of 0.25 will be statistically significant if the adoption study includes at least 45 parent-offspring pairs. Such a result would indicate that it is highly likely (95 percent probability) that the true correlation is greater than zero.

The question about how much genetics contributes to a trait refers to effect size, the extent to which individual differences for the trait in the population can be accounted for by genetic differences among individuals. Effect size in this sense refers to individual differences for a trait in the entire population, not to certain individuals. For example, if PKU were left untreated, it would have a huge effect on the cognitive development of individuals homozygous for the recessive allele. However, because such individuals represent only 1 in 10,000 individuals in the population, this huge effect for these few individuals would have little effect overall on the variation in cognitive ability in the entire population. Thus, the size of the effect of PKU in the population is very small.

Many statistically significant environmental effects in the behavioral sciences involve very small effects in the population. For example, birth order is significantly related to intelligence test (IQ) scores (first-born children have higher IQs). This is a small effect in that the mean difference between first- and second-born siblings is less than two IQ points and their IQ distributions almost completely overlap. Birth order accounts for about 1 percent of the variance of IQ scores when other factors are controlled. In other words, if all you know about two siblings is their birth order, then you know practically nothing about their IQs.

In contrast, genetic effect sizes are often very large, among the largest effects found in the behavioral sciences, accounting for as much as half of the variance. **The statistic that estimates the genetic effect size is called heritability. Heritability is the proportion of phenotypic variance that can be accounted for by genetic differences among individuals.** As explained in the Appendix, heritability can be estimated from the correlations for relatives. For example, if the correlation for “genetic” (adopted apart) relatives is zero, then heritability is zero. For first-degree “genetic” relatives, their correlation reflects half of the effect of genes because they are only 50 percent similar genetically. That is, if heritability is 100 percent, their correlation would be 0.50. In Figure 6.2, the correlation for “genetic” (adopted-apart) siblings is 0.24 for IQ^ scores. Doubling this correlation yields a heritability estimate of 48 percent, which suggests that about half of the variance in IQ scores can be explained by genetic differences among individuals. (pp. 86-87)

It's the sun

"Over the past few hundred years, there has been a steady increase in the numbers of sunspots, at the time when the Earth has been getting warmer. The data suggests solar activity is influencing the global climate causing the world to get warmer." (BBC)