Optical depth

From Wikipedia, the free encyclopedia

 

In physics, optical depth or optical thickness, is the natural logarithm of the ratio of incident to transmitted radiant power through a material, and spectral optical depth or spectral optical thickness is the natural logarithm of the ratio of incident to transmitted spectral radiant power through a material.^[1] Optical depth is dimensionless, and in particular is not a length, though it is a monotonically increasing function of path length, and approaches zero as the path length approaches zero. The use of the term "optical density" for optical depth is discouraged.^[1]

In chemistry, a closely related quantity called "absorbance" or "decadic absorbance" is used instead of optical depth: the common logarithm of the ratio of incident to transmitted radiant power through a material, that is the optical depth divided by ln 10.

Mathematical definitions

Optical depth

Optical depth of a material, denoted τ, is given by:^[2]

where

• Φ_e^t is the radiant flux transmitted by that material;
• Φ_eⁱ is the radiant flux received by that material;
• T is the transmittance of that material.

Absorbance is related to optical depth by:

where A is the absorbance.

Spectral optical depth

Spectral absorbance in frequency and spectral absorbance in wavelength of a material, denoted τ_ν and τ_λ respectively, are given by:^[1]

where

• Φ_e,ν^t is the spectral radiant flux in frequency transmitted by that material;
• Φ_e,νⁱ is the spectral radiant flux in frequency received by that material;
• T_ν is the spectral transmittance in frequency of that material;
• Φ_e,λ^t is the spectral radiant flux in wavelength transmitted by that material;
• Φ_e,λⁱ is the spectral radiant flux in wavelength received by that material;
• T_λ is the spectral transmittance in wavelength of that material.

Spectral absorbance is related to spectral optical depth by:

where

• A_ν is the spectral abosrbance in frequency;
• A_λ is the spectral absorbance in wavelength.

Relationship with attenuation

Attenuance

Optical depth measures the attenuation of the transmitted radiant power in a material. Attenuation can be caused by absorption, but also reflection, scattering, and other physical processes. Optical depth of a material is approximately equal to its attenuance when both the absorbance is much less than 1 and the emittance of that material (not to be confused with radiant exitance or emissivity) is much less than the optical depth:

where

• Φ_e^t is the radiant power transmitted by that material;
• Φ_e^att is the radiant power attenuated by that material;
• Φ_eⁱ is the radiant power received by that material;
• Φ_e^e is the radiant power emitted by that material;
• T = Φ_e^t/Φ_eⁱ is the transmittance of that material;
• ATT = Φ_e^att/Φ_eⁱ is the attenuance of that material;
• E = Φ_e^e/Φ_eⁱ is the emittance of that material,

and according to Beer–Lambert law,

so:

Attenuation coefficient

Optical depth of a material is also related to its attenuation coefficient by:

where

• l is the thickness of that material through which the light travels;
• α(z) is the attenuation coefficient or Napierian attenuation coefficient of that material at z,

and if α(z) is uniform along the path, the attenuation is said to be a linear attenuation and the relation becomes:

Sometimes the relation is given using the attenuation cross section of the material, that is its attenuation coefficient divided by its number density:

where

• σ is the attenuation cross section of that material;
• N(z) is the number density of that material at z,

and if N(z) is uniform along the path, the relation becomes:

Applications

Atomic physics

In atomic physics, the spectral optical depth of a cloud of atoms can be calculated from the quantum-mechanical properties of the atoms. It is given by

where

• d is the transition dipole moment;
• N is the number of atoms;
• ν is the frequency of the beam;
• c is the speed of light;
• ħ is Planck's constant;
• ε₀ is the vacuum permittivity;
• σ the cross section of the beam;
• γ the natural linewidth of the transition.

Atmospheric sciences

In atmospheric sciences, one often refers to the optical depth of the atmosphere as corresponding to the vertical path from Earth's surface to outer space; at other times the optical path is from the observer's altitude to outer space. The optical depth for a slant path is τ = mτ′, where τ′ refers to a vertical path, m is called the relative airmass, and for a plane-parallel atmosphere it is determined as m = sec θ where θ is the zenith angle corresponding to the given path. Therefore,

The optical depth of the atmosphere can be divided into several components, ascribed to Rayleigh scattering, aerosols, and gaseous absorption. The optical depth of the atmosphere can be measured with a sun photometer.

Astronomy

In astronomy, the photosphere of a star is defined as the surface where its optical depth is 2/3. This means that each photon emitted at the photosphere suffers an average of less than one scattering before it reaches the observer. At the temperature at optical depth 2/3, the energy emitted by the star (the original derivation is for the Sun) matches the observed total energy emitted.^{[citation needed][clarification needed]}

Note that the optical depth of a given medium will be different for different colors (wavelengths) of light.

For planetary rings, the optical depth is the (negative logarithm of the) proportion of light blocked by the ring when it lies between the source and the observer. This is usually obtained by observation of stellar occultations.

SI radiometry units

Quantity		Unit		Dimension	Notes
Name	Symbol[nb 1]	Name	Symbol	Symbol
Radiant energy	Qe[nb 2]	joule	J	M⋅L2⋅T−2	Energy of electromagnetic radiation.
Radiant energy density	we	joule per cubic metre	J/m3	M⋅L−1⋅T−2	Radiant energy per unit volume.
Radiant flux	Φe[nb 2]	watt	W or J/s	M⋅L2⋅T−3	Radiant energy emitted, reflected, transmitted or received, per unit time. This is sometimes also called "radiant power".
Spectral flux	Φe,ν[nb 3] or Φe,λ[nb 4]	watt per hertz or watt per metre	W/Hz or W/m	M⋅L2⋅T−2 or M⋅L⋅T−3	Radiant flux per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅m−2⋅nm−1.
Radiant intensity	Ie,Ω[nb 5]	watt per steradian	W/sr	M⋅L2⋅T−3	Radiant flux emitted, reflected, transmitted or received, per unit solid angle. This is a directional quantity.
Spectral intensity	Ie,Ω,ν[nb 3] or Ie,Ω,λ[nb 4]	watt per steradian per hertz or watt per steradian per metre	W⋅sr−1⋅Hz−1 or W⋅sr−1⋅m−1	M⋅L2⋅T−2 or M⋅L⋅T−3	Radiant intensity per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅m−2⋅nm−1. This is a directional quantity.
Radiance	Le,Ω[nb 5]	watt per steradian per square metre	W⋅sr−1⋅m−2	M⋅T−3	Radiant flux emitted, reflected, transmitted or received by a surface, per unit solid angle per unit projected area. This is a directional quantity. This is sometimes also confusingly called "intensity".
Spectral radiance	Le,Ω,ν[nb 3] or Le,Ω,λ[nb 4]	watt per steradian per square metre per hertz or watt per steradian per square metre, per metre	W⋅sr−1⋅m−2⋅Hz−1 or W⋅sr−1⋅m−3	M⋅T−2 or M⋅L−1⋅T−3	Radiance of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅m−2⋅nm−1. This is a directional quantity. This is sometimes also confusingly called "spectral intensity".
Irradiance	Ee[nb 2]	watt per square metre	W/m2	M⋅T−3	Radiant flux received by a surface per unit area. This is sometimes also confusingly called "intensity".
Spectral irradiance	Ee,ν[nb 3] or Ee,λ[nb 4]	watt per square metre per hertz or watt per square metre, per metre	W⋅m−2⋅Hz−1 or W/m3	M⋅T−2 or M⋅L−1⋅T−3	Irradiance of a surface per unit frequency or wavelength. The terms spectral flux density or more confusingly "spectral intensity" are also used. Non-SI units of spectral irradiance include Jansky = 10−26 W⋅m−2⋅Hz−1 and solar flux unit (1SFU = 10−22 W⋅m−2⋅Hz−1).
Radiosity	Je[nb 2]	watt per square metre	W/m2	M⋅T−3	Radiant flux leaving (emitted, reflected and transmitted by) a surface per unit area. This is sometimes also confusingly called "intensity".
Spectral radiosity	Je,ν[nb 3] or Je,λ[nb 4]	watt per square metre per hertz or watt per square metre, per metre	W⋅m−2⋅Hz−1 or W/m3	M⋅T−2 or M⋅L−1⋅T−3	Radiosity of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅m−2⋅nm−1. This is sometimes also confusingly called "spectral intensity".
Radiant exitance	Me[nb 2]	watt per square metre	W/m2	M⋅T−3	Radiant flux emitted by a surface per unit area. This is the emitted component of radiosity. "Radiant emittance" is an old term for this quantity. This is sometimes also confusingly called "intensity".
Spectral exitance	Me,ν[nb 3] or Me,λ[nb 4]	watt per square metre per hertz or watt per square metre, per metre	W⋅m−2⋅Hz−1 or W/m3	M⋅T−2 or M⋅L−1⋅T−3	Radiant exitance of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅m−2⋅nm−1. "Spectral emittance" is an old term for this quantity. This is sometimes also confusingly called "spectral intensity".
Radiant exposure	He	joule per square metre	J/m2	M⋅T−2	Radiant energy received by a surface per unit area, or equivalently irradiance of a surface integrated over time of irradiation. This is sometimes also called "radiant fluence".
Spectral exposure	He,ν[nb 3] or He,λ[nb 4]	joule per square metre per hertz or joule per square metre, per metre	J⋅m−2⋅Hz−1 or J/m3	M⋅T−1 or M⋅L−1⋅T−2	Radiant exposure of a surface per unit frequency or wavelength. The latter is commonly measured in J⋅m−2⋅nm−1. This is sometimes also called "spectral fluence".
Hemispherical emissivity	ε			1	Radiant exitance of a surface, divided by that of a black body at the same temperature as that surface.
Spectral hemispherical emissivity	εν or ελ			1	Spectral exitance of a surface, divided by that of a black body at the same temperature as that surface.
Directional emissivity	εΩ			1	Radiance emitted by a surface, divided by that emitted by a black body at the same temperature as that surface.
Spectral directional emissivity	εΩ,ν or εΩ,λ			1	Spectral radiance emitted by a surface, divided by that of a black body at the same temperature as that surface.
Hemispherical absorptance	A			1	Radiant flux absorbed by a surface, divided by that received by that surface. This should not be confused with "absorbance".
Spectral hemispherical absorptance	Aν or Aλ			1	Spectral flux absorbed by a surface, divided by that received by that surface. This should not be confused with "spectral absorbance".
Directional absorptance	AΩ			1	Radiance absorbed by a surface, divided by the radiance incident onto that surface. This should not be confused with "absorbance".
Spectral directional absorptance	AΩ,ν or AΩ,λ			1	Spectral radiance absorbed by a surface, divided by the spectral radiance incident onto that surface. This should not be confused with "spectral absorbance".
Hemispherical reflectance	R			1	Radiant flux reflected by a surface, divided by that received by that surface.
Spectral hemispherical reflectance	Rν or Rλ			1	Spectral flux reflected by a surface, divided by that received by that surface.
Directional reflectance	RΩ			1	Radiance reflected by a surface, divided by that received by that surface.
Spectral directional reflectance	RΩ,ν or RΩ,λ			1	Spectral radiance reflected by a surface, divided by that received by that surface.
Hemispherical transmittance	T			1	Radiant flux transmitted by a surface, divided by that received by that surface.
Spectral hemispherical transmittance	Tν or Tλ			1	Spectral flux transmitted by a surface, divided by that received by that surface.
Directional transmittance	TΩ			1	Radiance transmitted by a surface, divided by that received by that surface.
Spectral directional transmittance	TΩ,ν or TΩ,λ			1	Spectral radiance transmitted by a surface, divided by that received by that surface.
Hemispherical attenuation coefficient	μ	reciprocal metre	m−1	L−1	Radiant flux absorbed and scattered by a volume per unit length, divided by that received by that volume.
Spectral hemispherical attenuation coefficient	μν or μλ	reciprocal metre	m−1	L−1	Spectral radiant flux absorbed and scattered by a volume per unit length, divided by that received by that volume.
Directional attenuation coefficient	μΩ	reciprocal metre	m−1	L−1	Radiance absorbed and scattered by a volume per unit length, divided by that received by that volume.
Spectral directional attenuation coefficient	μΩ,ν or μΩ,λ	reciprocal metre	m−1	L−1	Spectral radiance absorbed and scattered by a volume per unit length, divided by that received by that volume.

Eugenics

From Wikipedia, the free encyclopedia

Logo from the Second International Eugenics Conference, 1921, depicting eugenics as a tree which unites a variety of different fields.^[1]

Eugenics (/juːˈdʒɛnɪks/; from Greek εὐγενής eugenes "well-born" from εὖ eu, "good, well" and γένος genos, "race, stock, kin")^[2][3] is a set of beliefs and practices that aims at improving the genetic quality of the human population.^[4][5] It is a social philosophy advocating the improvement of human genetic traits through the promotion of higher rates of sexual reproduction for people with desired traits (positive eugenics), or reduced rates of sexual reproduction and sterilization of people with less-desired or undesired traits (negative eugenics), or both.^[6] Alternatively, gene selection rather than "people selection" has recently been made possible through advances in gene editing (e.g. CRISPR).^[7] The exact definition of eugenics has been a matter of debate since the term was coined. The definition of it as a "social philosophy"—that is, a philosophy with implications for social order—is not universally accepted, and was taken from Frederick Osborn's 1937 journal article "Development of a Eugenic Philosophy".^[6]

While eugenic principles have been practiced as far back in world history as Ancient Greece, the modern history of eugenics began in the early 20th century when a popular eugenics movement emerged in the United Kingdom^[8] and spread to many countries, including the United States and most European countries. In this period, eugenic ideas were espoused across the political spectrum. Consequently, many countries adopted eugenic policies meant to improve the genetic stock of their countries. Such programs often included both "positive" measures, such as encouraging individuals deemed particularly "fit" to reproduce, and "negative" measures such as marriage prohibitions and forced sterilization of people deemed unfit for reproduction. People deemed unfit to reproduce often included people with mental or physical disabilities, people who scored in the low ranges of different IQ tests, criminals and deviants, and members of disfavored minority groups. The eugenics movement became negatively associated with Nazi Germany and the Holocaust—the murder by the German state of approximately 11 million people—when many of the defendants at the Nuremberg trials attempted to justify their human rights abuses by claiming there was little difference between the Nazi eugenics programs and the US eugenics programs.^[9] In the decades following World War II, with the institution of human rights, many countries gradually abandoned eugenics policies, although some Western countries, among them Sweden and the US, continued to carry out forced sterilizations for several decades.

Since the 1980s and 1990s when new assisted reproductive technology procedures became available, such as gestational surrogacy (available since 1985), preimplantation genetic diagnosis (available since 1989) and cytoplasmic transfer (first performed in 1996), fear about a possible future revival of eugenics and a widening of the gap between the rich and the poor has emerged.

A major criticism of eugenics policies is that, regardless of whether "negative" or "positive" policies are used, they are vulnerable to abuse because the criteria of selection are determined by whichever group is in political power. Furthermore, negative eugenics in particular is considered by many to be a violation of basic human rights, which include the right to reproduction. Another criticism is that eugenic policies eventually lead to a loss of genetic diversity, resulting in inbreeding depression instead due to a low genetic variation.

History

Francis Galton was an early eugenicist, coining the term itself and popularizing the collocation of the words "nature and nurture".^[10]

The idea of eugenics to produce better human beings has existed at least since Plato suggested selective mating to produce a guardian class.^[11] The idea of eugenics to decrease the birth of inferior human beings has existed at least since William Goodell (1829-1894) advocated the castration and spaying of the insane.^[12][13]

However, the term "eugenics" to describe the modern concept of improving the quality of human beings born into the world was originally developed by Francis Galton. Galton had read his half-cousin Charles Darwin's theory of evolution, which sought to explain the development of plant and animal species, and desired to apply it to humans. Galton believed that desirable traits were hereditary based on biographical studies, Darwin strongly disagreed with his interpretation of the book.^[14] In 1883, one year after Darwin's death, Galton gave his research a name: eugenics.^[15] Throughout its recent history, eugenics has remained a controversial concept.^[16]

Eugenics became an academic discipline at many colleges and universities and received funding from many sources.^[17] Organisations formed to win public support, and modify opinion towards responsible eugenic values in parenthood, included the British Eugenics Education Society of 1907, and the American Eugenics Society of 1921. Both sought support from leading clergymen, and modified their message to meet religious ideals.^[18] Three International Eugenics Conferences presented a global venue for eugenists with meetings in 1912 in London, and in 1921 and 1932 in New York. Eugenic policies were first implemented in the early 1900s in the United States.^[19] It has roots in France, Germany, Great Britain, and the United States.^[20] Later, in the 1920s and 30s, the eugenic policy of sterilizing certain mental patients was implemented in other countries, including Belgium,^[21] Brazil,^[22] Canada,^[23] Japan and Sweden.^[24]

The scientific reputation of eugenics started to decline in the 1930s, a time when Ernst Rüdin used eugenics as a justification for the racial policies of Nazi Germany. Nevertheless, in Sweden the eugenics program continued until 1975.^[24] In addition to being practised in a number of countries, eugenics was internationally organized through the International Federation of Eugenics Organizations.^[25] Its scientific aspects were carried on through research bodies such as the Kaiser Wilhelm Institute of Anthropology, Human Heredity, and Eugenics,^[26] the Cold Spring Harbour Carnegie Institution for Experimental Evolution,^[27] and the Eugenics Record Office.^[28] Its political aspects involved advocating laws allowing the pursuit of eugenic objectives, such as sterilization laws.^[29] Its moral aspects included rejection of the doctrine that all human beings are born equal, and redefining morality purely in terms of genetic fitness.^[30] Its racist elements included pursuit of a pure "Nordic race" or "Aryan" genetic pool and the eventual elimination of "less fit" races.^[31][32]

As a social movement, eugenics reached its greatest popularity in the early decades of the 20th century. At this point in time, eugenics was practiced around the world and was promoted by governments and influential individuals and institutions. Many countries enacted^[33] various eugenics policies and programmes, including: genetic screening, birth control, promoting differential birth rates, marriage restrictions, segregation (both racial segregation and segregation of the mentally ill from the rest of the population), compulsory sterilization, forced abortions or forced pregnancies, and genocide. Most of these policies were later regarded as coercive or restrictive, and now few jurisdictions implement policies that are explicitly labelled as eugenic or unequivocally eugenic in substance. The methods of implementing eugenics varied by country; however, some early 20th century methods involved identifying and classifying individuals and their families, including the poor, mentally ill, blind, deaf, developmentally disabled, promiscuous women, homosexuals, and racial groups (such as the Roma and Jews in Nazi Germany) as "degenerate" or "unfit", the segregation or institutionalization of such individuals and groups, their sterilization, euthanasia, and their mass murder.^[34] The practice of euthanasia was carried out on hospital patients in the Aktion T4 centers such as Hartheim Castle.

Hartheim Euthanasia Centre in 2005

A Lebensborn birth house in Nazi Germany. Created with intention of raising the birth rate of "Aryan" children from extramarital relations of "racially pure and healthy" parents.

By the end of World War II, many of the discriminatory eugenics laws were largely abandoned, having become associated with Nazi Germany.^[34][35] After World War II, the practice of "imposing measures intended to prevent births within [a population] group" fell within the definition of the new international crime of genocide, set out in the Convention on the Prevention and Punishment of the Crime of Genocide.^[36] The Charter of Fundamental Rights of the European Union also proclaims "the prohibition of eugenic practices, in particular those aiming at selection of persons".^[37] In spite of the decline in discriminatory eugenics laws, government practices of compulsive sterilization continued into the 21st century. During the ten years President Alberto Fujimori led Peru from 1990 to 2000, allegedly 2,000 persons were involuntarily sterilized.^[38] China maintained its coercive one-child policy until 2015 as well as a suite of other eugenics based legislation in order to reduce population size and manage fertility rates of different populations.^[39][40][41] In 2007 the United Nations reported coercive sterilisations and hysterectomies in Uzbekistan.^[42] During the years 2005–06 to 2012–13, nearly one-third of the 144 California prison inmates who were sterilized did not give lawful consent to the operation.^[43]

Developments in genetic, genomic, and reproductive technologies at the end of the 20th century are raising numerous questions regarding the ethical status of eugenics, effectively creating a resurgence of interest in the subject. Some, such as UC Berkeley sociologist Troy Duster, claim that modern genetics is a back door to eugenics.^[44] This view is shared by White House Assistant Director for Forensic Sciences, Tania Simoncelli, who stated in a 2003 publication by the Population and Development Program at Hampshire College that advances in pre-implantation genetic diagnosis (PGD) are moving society to a "new era of eugenics", and that, unlike the Nazi eugenics, modern eugenics is consumer driven and market based, "where children are increasingly regarded as made-to-order consumer products".^[45] In a 2006 newspaper article, Richard Dawkins said that discussion regarding eugenics was inhibited by the shadow of Nazi misuse, to the extent that some scientists would not admit that breeding humans for certain abilities is at all possible. He believes that it is not physically different from breeding domestic animals for traits such as speed or herding skill. Dawkins felt that enough time had elapsed to at least ask just what the ethical differences were between breeding for ability versus training athletes or forcing children to take music lessons, though he could think of persuasive reasons to draw the distinction.^[46]

Some, such as Nathaniel C. Comfort from Johns Hopkins University, claim that the change from state-led reproductive-genetic decision-making to individual choice has moderated the worst abuses of eugenics by transferring the decision-making from the state to the patient and their family.^[47] Comfort suggests that "[t]he eugenic impulse drives us to eliminate disease, live longer and healthier, with greater intelligence, and a better adjustment to the conditions of society; and the health benefits, the intellectual thrill and the profits of genetic bio-medicine are too great for us to do otherwise."^[48] Others, such as bioethicist Stephen Wilkinson of Keele University and Honorary Research Fellow Eve Garrard at the University of Manchester, claim that some aspects of modern genetics can be classified as eugenics, but that this classification does not inherently make modern genetics immoral. In a co-authored publication by Keele University, they stated that "[e]ugenics doesn't seem always to be immoral, and so the fact that PGD, and other forms of selective reproduction, might sometimes technically be eugenic, isn't sufficient to show that they're wrong."^[49]

In October 2015, the United Nations' International Bioethics Committee wrote that the ethical problems of human genetic engineering should not be confused with the ethical problems of the 20th century eugenics movements; however, it is still problematic because it challenges the idea of human equality and opens up new forms of discrimination and stigmatization for those who do not want or cannot afford the enhancements.^[50]

Meanings and types

Karl Pearson (1912)

The term eugenics and its modern field of study were first formulated by Francis Galton in 1883,^[51] drawing on the recent work of his half-cousin Charles Darwin.^[52][53] Galton published his observations and conclusions in his book Inquiries into Human Faculty and Its Development.

The origins of the concept began with certain interpretations of Mendelian inheritance, and the theories of August Weismann.^[54] The word eugenics is derived from the Greek word eu ("good" or "well") and the suffix -genēs ("born"), and was coined by Galton in 1883 to replace the word "stirpiculture", which he had used previously but which had come to be mocked due to its perceived sexual overtones.^[55] Galton defined eugenics as "the study of all agencies under human control which can improve or impair the racial quality of future generations".^[56] Galton did not understand the mechanism of inheritance.^[57]

Eugenics has, from the very beginning, meant many different things.^{[citation needed]} Historically, the term has referred to everything from prenatal care for mothers to forced sterilization and euthanasia.^{[citation needed]} To population geneticists, the term has included the avoidance of inbreeding without altering allele frequencies; for example, J. B. S. Haldane wrote that "the motor bus, by breaking up inbred village communities, was a powerful eugenic agent."^[58] Debate as to what exactly counts as eugenics has continued to the present day.^[59]

Edwin Black, journalist and author of War Against the Weak, claims eugenics is often deemed a pseudoscience because what is defined as a genetic improvement of a desired trait is often deemed a cultural choice rather than a matter that can be determined through objective scientific inquiry.^[60] The most disputed aspect of eugenics has been the definition of "improvement" of the human gene pool, such as what is a beneficial characteristic and what is a defect. This aspect of eugenics has historically been tainted with scientific racism.

Early eugenists were mostly concerned with perceived intelligence factors that often correlated strongly with social class. Some of these early eugenists include Karl Pearson and Walter Weldon, who worked on this at the University College London.^[14]

Eugenics also had a place in medicine. In his lecture "Darwinism, Medical Progress and Eugenics", Karl Pearson said that everything concerning eugenics fell into the field of medicine. He basically placed the two words as equivalents. He was supported in part by the fact that Francis Galton, the father of eugenics, also had medical training.^[61]

Eugenic policies have been conceptually divided into two categories. Positive eugenics is aimed at encouraging reproduction among the genetically advantaged; for example, the reproduction of the intelligent, the healthy, and the successful.^[62] Possible approaches include financial and political stimuli, targeted demographic analyses, in vitro fertilization, egg transplants, and cloning.^[63] The movie Gattaca provides a fictional example of positive eugenics done voluntarily. Negative eugenics aimed to eliminate, through sterilization or segregation, those deemed physically, mentally, or morally "undesirable".^[62] This includes abortions, sterilization, and other methods of family planning.^[63] Both positive and negative eugenics can be coercive; abortion for fit women, for example, was illegal in Nazi Germany.^[64]

Jon Entine claims that eugenics simply means "good genes" and using it as synonym for genocide is an "all-too-common distortion of the social history of genetics policy in the United States." According to Entine, eugenics developed out of the Progressive Era and not "Hitler's twisted Final Solution".^[65]

Implementation methods

According to Richard Lynn, eugenics may be divided into two main categories based on the ways in which the methods of eugenics can be applied.^[66]

1. Classical Eugenics
   1. Negative eugenics by provision of information and services, i.e. reduction of unplanned pregnancies and births.^[67]
      1. "Just say no" campaigns.^[68]
      2. Sex education in schools.^[69]
      3. School-based clinics.^[70]
      4. Promoting the use of contraception.^[71]
      5. Emergency contraception.^[72]
      6. Research for better contraceptives.^[73]
      7. Sterilization.^[74]
      8. Abortion.^[75]
   2. Negative eugenics by incentives, coercion and compulsion.^[76]
      1. Incentives for sterilization.^[77]
      2. The Denver Dollar-a-day program, i.e. paying teenage mothers for not becoming pregnant again.^[78]
      3. Incentives for women on welfare to use contraceptions.^[79]
      4. Payments for sterilization in developing countries.^[80]
      5. Curtailment of benefits to welfare mothers.^[81]
      6. Sterilization of the "mentally retarded".^[82]
      7. Sterilization of female criminals.^[83]
      8. Sterilization of male criminals.^[84]
   3. Licences for parenthood.^[85][86][87]
   4. Positive eugenics.^[88]
      1. Financial incentives to have children.^[89]
      2. Selective incentives for childbearing.^[90]
      3. Taxation of the childless.^[91]
      4. Ethical obligations of the elite.^{[92][clarification needed]}
      5. Eugenic immigration.^[93]
2. New Eugenics
   1. Artificial insemination by donor.^[94]
   2. Egg donation.^[95]
   3. Prenatal diagnosis of genetic disorders and pregnancy terminations of defective fetuses.^[96]
   4. Embryo selection.^[97]
   5. Genetic engineering.^[98]
   6. Gene therapy.^[99]
   7. Cloning.^[100]

Arguments

Doubts on traits triggered by inheritance

The first major challenge to conventional eugenics based upon genetic inheritance was made in 1915 by Thomas Hunt Morgan, who demonstrated the event of genetic mutation occurring outside of inheritance involving the discovery of the hatching of a fruit fly (Drosophila melanogaster) with white eyes from a family of red-eyes.^[101] Morgan claimed that this demonstrated that major genetic changes occurred outside of inheritance and that the concept of eugenics based upon genetic inheritance was not completely scientifically accurate.^[101] Additionally, Morgan criticized the view that subjective traits, such as intelligence and criminality, were caused by heredity because he believed that the definitions of these traits varied and that accurate work in genetics could only be done when the traits being studied were accurately defined.^[102] In spite of Morgan's public rejection of eugenics, much of his genetic research was absorbed by eugenics.^[103][104]

Ethics

A common criticism of eugenics is that "it inevitably leads to measures that are unethical".^[105] Historically, this statement is evidenced by the obvious control of one group imposing its agenda on minority groups. This includes programs in England, Germany, and America targeting various groups, including Jews, homosexuals, Muslims, Romani, the homeless, and those with intellectual disabilities.^[106]

Many of the ethical concerns from eugenics arise from the controversial past, prompting a discussion on what place, if any, it should have in the future. Advances in science have changed eugenics. In the past, eugenics has had more to do with sterilization and enforced reproduction laws (i.e. no inter-racial marriage and marriage restrictions based on land ownership).^[107] Now, in the age of a progressively mapped genome, embryos can be tested for susceptibility to disease, gender, and genetic defects, and alternative methods of reproduction such as in vitro fertilization are becoming more common.^[108] In short, eugenics is no longer ex post facto regulation of the living but instead preemptive action on the unborn.

With this change, however, there are ethical concerns which lack adequate attention, and which must be addressed before eugenic policies can be properly implemented in the future. Sterilized individuals, for example, could volunteer for the procedure, albeit under incentive or duress, or at least voice their opinion. The unborn fetus on which these new eugenic procedures are performed cannot speak out, as the fetus lacks the voice to consent or to express his or her opinion.^[109] The ability to manipulate a fetus and determine who the child will be is something questioned by many of the opponents of, and even proponents for, eugenic policies.

Societal and political consequences of eugenics call for a place in the discussion on the ethics behind the eugenics movement.^[110] Public policy often focuses on issues related to race and gender, both of which could be controlled by manipulation of embryonic genes; eugenics and political issues are interconnected and the political aspect of eugenics must be addressed. Laws controlling the subjects, the methods, and the extent of eugenics will need to be considered in order to prevent the repetition of the unethical events of the past.

Most of the ethical concerns about eugenics involve issues of morality and power. Decisions about the morality and the control of this new science (and the subsequent results of the science) will need to be made as eugenics continue to influence the development of the science and medical fields.

Losing genetic diversity by classifying traits as diseases

Eugenic policies could also lead to loss of genetic diversity, in which case a culturally accepted "improvement" of the gene pool could very likely—as evidenced in numerous instances in isolated island populations (e.g., the dodo, Raphus cucullatus, of Mauritius)—result in extinction due to increased vulnerability to disease, reduced ability to adapt to environmental change, and other factors both known and unknown. A long-term species-wide eugenics plan might lead to a scenario similar to this because the elimination of traits deemed undesirable would reduce genetic diversity by definition.^[111]
Edward M. Miller claims that, in any one generation, any realistic program should make only minor changes in a fraction of the gene pool, giving plenty of time to reverse direction if unintended consequences emerge, reducing the likelihood of the elimination of desirable genes.^[112] Miller also argues that any appreciable reduction in diversity is so far in the future that little concern is needed for now.^[112]

While the science of genetics has increasingly provided means by which certain characteristics and conditions can be identified and understood, given the complexity of human genetics, culture, and psychology there is at this point no agreed objective means of determining which traits might be ultimately desirable or undesirable. Some diseases such as sickle-cell disease and cystic fibrosis respectively confer immunity to malaria and resistance to cholera when a single copy of the recessive allele is contained within the genotype of the individual. Reducing the instance of sickle-cell disease genes in Africa where malaria is a common and deadly disease could indeed have extremely negative net consequences.

However, some genetic diseases such as haemochromatosis can increase susceptibility to illness, cause physical deformities, and other dysfunctions, which provides some incentive for people to re-consider some elements of eugenics.

Autistic people have advocated a shift in perception of autism spectrum disorders as complex syndromes rather than diseases that must be cured. Proponents of this view reject the notion that there is an "ideal" brain configuration and that any deviation from the norm is pathological; they promote tolerance for what they call neurodiversity.^[113] Baron-Cohen argues that the genes for Asperger's combination of abilities have operated throughout recent human evolution and have made remarkable contributions to human history.^[114] The possible reduction of autism rates through selection against the genetic predisposition to autism is a significant political issue in the autism rights movement, which claims that autism is a part of neurodiversity.

Many culturally Deaf people oppose attempts to cure deafness, believing instead deafness should be considered a defining cultural characteristic not a disease.^{[115][116][117]} Some people have started advocating the idea that deafness brings about certain advantages, often termed "Deaf Gain."^[118][119]

Heterozygous recessive traits

The heterozygote test is used for the early detection of recessive hereditary diseases, allowing for couples to determine if they are at risk of passing genetic defects to a future child.^[120] The goal of the test is to estimate the likelihood of passing the hereditary disease to future descendants.^[120]

Recessive traits can be severely reduced, but never eliminated unless the complete genetic makeup of all members of the pool was known, as aforementioned. As only very few undesirable traits, such as Huntington's disease, are dominant, it could be argued^{[by whom?]} from certain perspectives that the practicality of "eliminating" traits is quite low.^{[citation needed]}

There are examples of eugenic acts that managed to lower the prevalence of recessive diseases, although not influencing the prevalence of heterozygote carriers of those diseases. The elevated prevalence of certain genetically transmitted diseases among the Ashkenazi Jewish population (Tay–Sachs, cystic fibrosis, Canavan's disease, and Gaucher's disease), has been decreased in current populations by the application of genetic screening.^[121]

Pleiotropic genes

Pleiotropy occurs when one gene influences multiple, seemingly unrelated phenotypic traits, an example being phenylketonuria, which is a human disease that affects multiple systems but is caused by one gene defect.^[122] Andrzej Pękalski, from the University of Wrocław, argues that eugenics can cause harmful loss of genetic diversity if a eugenics program selects for a pleiotropic gene that is also associated with a positive trait. Pekalski uses the example of a coercive government eugenics program that prohibits people with myopia from breeding but has the unintended consequence of also selecting against high intelligence since the two go together.^[123]

Supporters and critics

G. K. Chesterton, an opponent of eugenics, in 1905, by photographer Alvin Langdon Coburn

At its peak of popularity, eugenics was supported by a wide variety of prominent people, including Winston Churchill,^[124] Margaret Sanger,^[125][126] Marie Stopes,^[127][128] H. G. Wells,^[129] Norman Haire, Havelock Ellis, Theodore Roosevelt, Herbert Hoover, George Bernard Shaw, John Maynard Keynes, John Harvey Kellogg, Robert Andrews Millikan,^[130] Linus Pauling,^[131] Sidney Webb,^{[132][133][134]} and W. E. B. Du Bois.^[135]

In 1909 the Anglican clergymen William Inge and James Peile both wrote for the British Eugenics Education Society. Inge was an invited speaker at the 1921 International Eugenics Conference, which was also endorsed by the Roman Catholic Archbishop of New York Patrick Joseph Hayes.^[18] In 1925 Adolf Hitler praised and incorporated eugenic ideas in Mein Kampf and emulated eugenic legislation for the sterilization of "defectives" that had been pioneered in the United States.^[136]

Early critics of the philosophy of eugenics included the American sociologist Lester Frank Ward,^[137] the English writer G. K. Chesterton, the German-American anthropologist Franz Boas,^[138] and Scottish tuberculosis pioneer and author Halliday Sutherland. Ward's 1913 article "Eugenics, Euthenics, and Eudemics", Chesterton's 1917 book Eugenics and Other Evils, and Boas' 1916 article "Eugenics" (published in The Scientific Monthly) were all harshly critical of the rapidly growing movement. Sutherland identified eugenists as a major obstacle to the eradication and cure of tuberculosis in his 1917 address "Consumption: Its Cause and Cure",^[139] and criticism of eugenists and Neo-Malthusians in his 1921 book Birth Control led to a writ for libel from the eugenist Marie Stopes. Several biologists were also antagonistic to the eugenics movement, including Lancelot Hogben.^[140] Other biologists such as J. B. S. Haldane and R. A. Fisher expressed skepticism that sterilization of "defectives" would lead to the disappearance of undesirable genetic traits.^[141]

Some supporters of eugenics later reversed their positions on it. For example, H. G. Wells, who had called for "the sterilization of failures" in 1904,^[129] stated in his 1940 book The Rights of Man: Or What are we fighting for? that among the human rights he believed should be available to all people was "a prohibition on mutilation, sterilization, torture, and any bodily punishment".^[142]

Among institutions, the Catholic Church was an opponent of state-enforced sterilizations.^[143] Attempts by the Eugenics Education Society to persuade the British government to legalise voluntary sterilisation were opposed by Catholics and by the Labour Party.^{[page needed]} The American Eugenics Society initially gained some Catholic supporters, but Catholic support declined following the 1930 papal encyclical Casti connubii.^[18] In this, Pope Pius XI explicitly condemned sterilization laws: "Public magistrates have no direct power over the bodies of their subjects; therefore, where no crime has taken place and there is no cause present for grave punishment, they can never directly harm, or tamper with the integrity of the body, either for the reasons of eugenics or for any other reason."^[144]