Editing Nature versus nurture (section)

==Heritability estimates==
{{Main|Heritability}}
[[File:Sibling-correlation-422.png|422px|thumb|right|This chart illustrates three patterns one might see when studying the influence of genes and environment on traits in individuals. Trait A shows a high sibling correlation, but little heritability (i.e. high shared environmental variance ''c''<sup>2</sup>; low heritability ''h''<sup>2</sup>). Trait B shows a high heritability since the correlation of trait rises sharply with the degree of genetic similarity. Trait C shows low heritability, but also low correlations generally; this means Trait C has a high nonshared environmental variance ''e''<sup>2</sup>. In other words, the degree to which individuals display Trait C has little to do with either genes or broadly predictable environmental factors—roughly, the outcome approaches random for an individual. Notice also that even identical twins raised in a common family rarely show 100% trait correlation.]]

The term ''heritability'' refers only to the degree of genetic variation between people on a trait.  It does not refer to the degree to which a trait of a particular individual is due to environmental or genetic factors. The traits of an individual are always a complex interweaving of both.<ref name="PinkerBlankSlate">[[Steven Pinker|Pinker, Steven]]. 2002. ''[[The Blank Slate|The Blank Slate: The Modern Denial of Human Nature]]'' (1st ed.). [[Viking Press]]. {{ISBN|978-0670031511}}. [https://web.archive.org/web/20141226054452/http://net.educause.edu/ir/library/pdf/ff0616S.pdf Lay summary].</ref> For an individual, even strongly genetically influenced, or "obligate" traits, such as eye color, assume the inputs of a typical environment during ontogenetic development (e.g., certain ranges of temperatures, oxygen levels, etc.).

In contrast, the "heritability index" statistically quantifies the extent to which variation ''between individuals'' on a trait is due to variation in the genes those individuals carry. In animals where breeding and environments can be controlled experimentally, heritability can be determined relatively easily. Such experiments would be unethical for human research. This problem can be overcome by finding existing populations of humans that reflect the experimental setting the researcher wishes to create.

One way to determine the contribution of genes and environment to a trait is to [[twin study|study twins]].<ref>{{cite web |title=Have Researchers Finally Settled The Nature Vs Nurture Debate? |date=19 May 2015 |url=https://www.iflscience.com/health-and-medicine/have-researchers-settled-nature-vs-nurture-debate/}}</ref> In one kind of study, [[identical twins]] reared apart are compared to randomly selected pairs of people. The twins share identical genes, but different family environments. Twins reared apart are not assigned at random to foster or adoptive parents. In another kind of twin study, identical twins reared together (who share family environment and genes) are compared to [[fraternal twins]] reared together (who also share family environment but only share half their genes). Another condition that permits the disassociation of genes and environment is [[adoption]]. In one kind of [[adoption study]], biological siblings reared together (who share the same family environment and half their genes) are compared to adoptive siblings (who share their family environment but none of their genes).

In many cases, it has been found that genes make a substantial contribution, including psychological traits such as intelligence and personality.<ref>{{cite web |author=Neill, J. T. |year=2004 |url=http://www.wilderdom.com/personality/L4-1IntelligenceNatureVsNurture.html |title=Nature vs Nurture in Intelligence |website=wilderdom.com |access-date=2007-03-28 |archive-url=https://web.archive.org/web/20100825174434/http://wilderdom.com/personality/L4-1IntelligenceNatureVsNurture.html |archive-date=2010-08-25 |url-status=dead }}</ref> Yet heritability may differ in other circumstances, for instance environmental deprivation. Examples of low, medium, and high heritability traits include:

{|class="wikitable" style="margin:1em auto; width:60%;"
|-
!Low heritability!!Medium heritability!!High heritability
|-
|align=center|Specific language||align=center|Weight||align=center|Blood type
|-
|align=center|Specific religion||align=center|Religiosity||align=center|Eye color
|}

Twin and adoption studies have their methodological limits. For example, both are limited to the range of environments and genes which they sample.  Almost all of these studies are conducted in Western countries, and therefore cannot necessarily be extrapolated globally to include non-western populations.  Additionally, both types of studies depend on particular assumptions, such as the [[equal environments assumption]] in the case of twin studies, and the lack of pre-adoptive effects in the case of adoption studies.

Since the definition of "nature" in this context is tied to "heritability", the definition of "nurture" has consequently become very wide, including any type of causality that is not heritable. The term has thus moved away from its original connotation of "cultural influences" to include all effects of the environment, including; indeed, a substantial source of environmental input to [[human nature]] may arise from stochastic variations in prenatal development and is thus in no sense of the term "cultural".<ref name="pmid9331275">{{cite journal |last1=Rice|first1=D. S. |last2=Tang|first2=Q. |last3=Williams|first3=R. W. |last4=Harris|first4=B.S. |last5=Davisson|first5=M. T. |last6=Goldowitz|first6=D. |title=Decreased retinal ganglion cell number and misdirected axon growth associated with fissure defects in ''Bst''/+ mutant mice |url=https://iovs.arvojournals.org/article.aspx?articleid=2161697 |journal=Investigative Ophthalmology & Visual Science |volume=38 |issue=10 |pages=2112–24 |date=September 1997 |pmid=9331275 }}</ref><ref name="pmid8476987">{{cite journal |last1=Stetter|first1=M. |last2=Lang|first2=E. W. |last3=Müller|first3=A. |title=Emergence of orientation selective simple cells simulated in deterministic and stochastic neural networks |journal=Biological Cybernetics |volume=68 |issue=5 |pages=465–76 |year=1993 |pmid=8476987 |doi=10.1007/BF00198779 |s2cid=3184341 |url=https://epub.uni-regensburg.de/17387/1/lang4.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://epub.uni-regensburg.de/17387/1/lang4.pdf |archive-date=2022-10-09 |url-status=live }}</ref>