It defines heritability as the extent to which genetic individual differences contribute to individual differences in observed behavior or phenotypic individual differences.
You should memorize both of these definitions. Because heritability is a proportion, its numerical value will range from 0. For human behavior, almost all estimates of heritability are in the moderate range of. The quantity 1. Environmentability has an analogous interpretation to heritability. Heritability can be difficult to understand, so there are many misconceptions about what it can and cannot tell us about a given trait:.
Heritability does not indicate what proportion of a trait is determined by genes and what proportion is determined by environment. So, a heritability of 0. Knowing the heritability of a trait does not provide information about which genes or environmental influences are involved, or how important they are in determining the trait. Heritable is not the same as familial.
A trait is described as familial if it is shared by members of a family. Traits can appear in families for many reasons in addition to genetics, such as similarities in lifestyle and environment.
For example, the language that is spoken tends to be shared in families, but it has no genetic contribution and so is not heritable. Heritability does not give any information about how easy or difficult it is to change a trait. For example, hair color is a trait with high heritability, but it is very easy to change with dye. If heritability provides such limited information, why do researchers study it? Heritability is of particular interest in understanding traits that are very complex with many contributing factors.
Moore DS, Shenk D. The heritability fallacy. Wiley Interdiscip Rev Cogn Sci. Epub Dec 1. So, if on a particular day humidity is very high at the North Pole and very low at the South Pole, snow will only fall at the North Pole.
This can fully account for the variation in snowfall between the North Pole and the South Pole on that day. However, this accounting for variation should not be taken to imply that low temperature is unimportant in causing snow; obviously temperature is a very important factor.
When a factor does not vary across situations, it cannot account for the variations in outcomes across those situations, but this does not mean that this constant factor is unimportant in causing the outcome. So, accounting for variation tells us little about causation.
This is also possibly true when explaining our traits. It is very possible that genetic factors account for 90 percent of the differences seen in people's heights, but this does not mean that genetic factors are necessarily more important than environmental factors in causing people's heights.
Twenty-five years ago, the scientist Richard Lewontin devised an insightful demonstration to show this. Imagine planting ordinary, genetically diverse seeds into two radically different environments and then allowing them to grow to their full heights. One environment is very deprived, with just barely enough light, nutrients, and water for survival.
The other environment is enriched with ideal amounts of light, water, and nutrients. All of the variation in height within each tray must be due to the genetic diversity of the seeds, since the seeds developed in identical environments, and therefore the variation observed in the heights of the plants within a tray cannot be attributed to differing environmental factors.
So regardless of the environments in which the plants grew, heritability is 1, or percent, within each tray. Yet obviously environmental factors played a large role in each individual tray, so even if heritability is percent, the environment can have very powerful effects on the appearance of a trait.
To restate: since the environment for the seeds within each tray is identical, the genetic diversity of the seeds must account for the observed height differences, so the heritability within each tray is very high, probably close to 1. Yet the heritability between the trays is very low since the environments are so disparate and the genetic material is similar. The large observed differences in height between the trays are due to environmental differences.
Another example, this one from Stephen Jay Gould, is illuminating. Think about the relative heights of men in a poor village in an underdeveloped country years ago. The average height for these malnourished men might be 5 feet 2 inches. The heritability in observed heights within this particular society can be quite high; men of tall fathers are on the average considerably taller than men of short fathers. However, this does not mean that a program of improved sanitation and nutrition could not significantly raise the average height of this group in a few generations.
This reflects one of the many significant errors of Herrnstein and Murray's controversial book, The Bell Curve. The error is to assume that genetic variation that can account for variation within a group is also the reason for the variation between groups. The converse is also true. Even if the heritability of a trait is 0, it can still be tremendously affected by genetic factors. Imagine taking a bunch of genetically identical seeds cloned if necessary and scattering them in a variety of soil types.
All the variation in the plants' heights would be accounted for by the environment since the seeds are genetically identical , so the heritability of height here is 0.
Yet, no one could reasonably argue that the genetic information in the seeds has no effect on the plants' heights. Compare dandelion seeds and those of the California redwoods. So a trait can be importantly affected by genetic factors even if it is not heritable at all. If both fully heritable and fully nonheritable traits can be significantly affected by environmental and genetic factors, then how do they differ?
In terms of the extent to which the environment and genes might affect traits, there is no difference. Heritability estimates tell us nothing about how genetic and environmental factors affect traits. So remember that when someone might mention or report that IQ is highly heritable; it does not mean or imply that environmental resources should not be employed to raise IQ scores.
To say that high heritability estimates of IQ scores are less open to environmental interventions is false. It is false because environmental interventions can greatly affect even maximally heritable traits. You might thus ask what the point or use is of heritability estimates. The answer is that they can estimate the sources of differences among people, but only for a particular population, at a particular time, and in particular circumstances.
Heritability estimates cannot be generalized, because as the situation changes, the estimates can change drastically. To illustrate this, think of the snowfall example again. When comparing the variation in snowfall between the North and South Poles, the relative humidity is essential.
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