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Nature versus nurture
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Although "nurture" may have historically referred mainly to the care given to children by their parents, a number of other environmental factors probably also would count as "nurture" in a contemporary nature versus nurture debate, including one's childhood friends, one's early experiences with television, or even one's experience in the womb. Additionally, although childhood experience (especially early childhood experience) is often regarded as more influential in who one becomes than post-childhood experience, a liberal interpretation of "nurture" might count all life experience as "nurture".
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In a few clear-cut cases, it makes sense to say that a trait is due almost entirely to nature, or almost entirely to nurture. In the case of Huntington's disease, nature seems to be the right answer; basically, you will get the disease if and only if you have the corresponding disease. In the case of which particular language you speak, nurture seems to be the right answer; linguists have found that any "normal" child can learn any human language. With most interesting traits, however, there is probably significant mixing between nature and nurture, and people may disagree wildly about the relative importance of the two.
How we can try to compare the effects of nature and nurture, and why this is hard
Much of current thinking tends to discount the notion of genetics as valid in determining subjectively qualified traits, such as intelligence or personality but research has indicated that genetics often influences the development of these traits. Identical twins raised separately, for example, have often been found to live similar lives and have similar personalities and levels of intelligence. It is also thought that the environment may trigger the expression of certain genes, that is, determine whether and to what extent a genetic predisposition will actually manifest itself. Hence, untangling nature and nurture, even with experiments like the above, can be very problematic and open to wide interpretation.
After a long, contentious, maturing, what can be said scientifically, is that for valid categorical attributes, there can be probabilities assigned to genetic triggers[?]. This is the limit of what genetics can scientifically "predict" about human psychological development.
A researcher seeking to quantify the influence of genes or environment on a trait needs to be able to separate the effects of one factor away from that of another. Often this reduces to calculating the heritability of a trait.
In many cases the difficulty of creating situations suitable for testing environmental and genetic influence on traits has been compensated for by finding existing populations that reflect the experimental setting the researcher wishes to create. For example, many twin studies have made use of identical twins (who have the same genetic makeup) who were raised in differing environments in order to control for genetic effects: that is, any variation between twins is clearly attributable to the environment, allowing the researcher to quantify the effects of the environment by measuring variance of a trait between twins.
Contemporary researchers have pointed out the likelihood that the individual, to some extent, shapes their own environment in ways that are presumably influenced by their genes. In addition, environment may trigger the expression of genes, that is, determine whether and to what extent a genetic predisposition will actually manifest itself. Hence, untangling nature and nurture, even with experiments like the above, can be almost impossible.
Another is the question of intelligence - was Einstein genetically predestined to become a revolutionary thinker? The subject is highly contentious, as many questions raised seem unlikey to be answerable scientifically. With so many variables to contend with, it seems impossible to isolate the effects that either genes or the environment have on the subjects of experiments.
Moral difficulties: eugenics, etc..
Modern science, however, tends to frown upon giving too much weight to the nature side of the argument, in part because of social consciousness[?]. Historically, much of this debate has had undertones of racist, and eugenicist policies - the notion of race as a scientific validity has often been assumed as a prerequisite in various incarnations of the nature versus nurture debate. Genetics, long having been used as "scientific" justification for genocide, or race-based discrimination.
...Steven Pinker: moral ideals directing science?
Philosophical difficulties: are the traits real?
It is sometimes a question whether the "trait" being measured is even a real thing. Much energy has been devoted to calculating the heritability of intelligence (usually the I.Q., or intelligence quotient), but there is still far from any agreement on whether 'intelligence' exists, or how one should define or measure it.
Within the debates surrounding cloning, for example, is the far-fetched contention that a Jesus or a Hitler could be "re-created" through genetic cloning. Current thinking finds this largely preposterous, and discounts the possibility that the clone of anyone would grow up to be the same individual.
A number of social issues exist, especially in education and in law with regards to culpability.
The concurrent development phenomenon: identical twins, separated at birth, grow to look and act so similarly?
Some nature versus nurture debates are criticized for leaving little role for free will; if "nature" and "nurture" together have so much influence on who I am, then is there such a thing as "free will"?
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