Sexual Reform Movements in Europe and the Reaction of the 1930s (1885–1941)
How far did the sexual reform movements that arose from the late nineteenth century in Germany, Britain, Soviet Russia and Spain get against laws punishing homosexuality, abortion and marriage? And why did movements that took one another as reference across borders in the 1920s collapse at almost the same time in the 1930s, in Berlin, Moscow and Madrid?
Growing out of late nineteenth-century sexology, neo-Malthusianism and socialist theories of the family, the sexual reform movements fought laws such as Germany’s Paragraphs 175 and 218 and Britain’s Labouchère Amendment. In the 1920s Soviet Russia legalised abortion and dropped its sodomy law, and Berlin’s Institute for Sexual Science and the World League for Sexual Reform of 1928 linked activists in Germany, Britain, the Soviet Union and Spain. Yet the movements were divided over homosexuality, abortion and eugenics, and left-wing support wavered.


Nineteenth-century roots: sexology, neo-Malthusianism and the socialist theory of the family
The European sexual reform movements began by making sex an object of science. In 1862 the German jurist Karl Heinrich Ulrichs declared himself an “Urning” and explained love between men as inborn and natural, and on 29 August 1867, at the congress of German jurists in Munich, he became the first homosexual to defend homosexuality in public. Richard von Krafft-Ebing’s Psychopathia Sexualis (1886) is regarded as the work that established sexology as a discipline, and in England Havelock Ellis’s Sexual Inversion (1897) offered the first objective study of homosexuality, treating it as neither disease nor crime. The same year Magnus Hirschfeld founded the Scientific-Humanitarian Committee in Berlin, tying sexology to the campaign to change the penal law. 1 2 3 4
The Scientific-Humanitarian Committee was founded on 15 May 1897, four days before Oscar Wilde’s release from prison, in Hirschfeld’s flat with the publisher Max Spohr and others. After 1919 it was closely tied to the Institute for Sexual Science and took over its theories, all of which constructed homosexuals as a biological “third sex” between man and woman. The strategy of proving homosexuality inborn so as to make the penal law inapplicable came under criticism inside and outside the movement already in the Weimar years, the starting point of the conflicts within the movement, involving the Männerbund theory and Freudo-Marxism, that the event on the German sexual reform movement follows in detail. 4
Another current that made sex and reproduction a social question was neo-Malthusianism. Unlike Malthus, it advocated contraception rather than abstinence, and in Britain it produced organisations such as the Malthusian League. In 1877 Annie Besant and Charles Bradlaugh were prosecuted for publishing Fruits of Philosophy, a book by the American birth-control campaigner Charles Knowlton, and the trial made birth control a public topic. In 1900 the Universal Federation of the League of Human Regeneration, a neo-Malthusian international, was founded in secret in the Paris home of the Catalan anarchist Francisco Ferrer Guardia, and in 1904 its Spanish section was set up in Barcelona. In Germany the League for the Protection of Mothers, led from 1905 by Helene Stöcker, called for the protection of unmarried mothers, sex education and contraception. 5 6 7 8
The socialist movement offered a theory of the family and sexuality as products of class society. Friedrich Engels’s The Origin of the Family, Private Property and the State (1884), based partly on Marx’s notes on Lewis H. Morgan’s Ancient Society, tied the monogamous family to private property. August Bebel’s Woman and Socialism (1879) was the work on sexuality most widely read by rank-and-file members of the German Social Democratic Party, and on 13 January 1898, bringing the petition to repeal Paragraph 175 before the Reichstag, Bebel became the first known politician to support homosexual emancipation openly. Yet in the same book he attributed “this crime against nature” to sexual indulgence and excess, a sign of the labour movement’s ambivalence. 9 10 11


The laws to fight: Paragraphs 175 and 218, the Labouchère Amendment and the tsarist code
What the German movement fought were two articles that came into force with the penal code of the North German Confederation on 1 January 1871. Paragraph 175 punished sex between men and had been used to convict some 140,000 men by its repeal in 1994; Paragraph 218 sent women who had abortions, and those who performed them, to prison. They were the targets of the homosexual emancipation movement and of the women’s and labour movements respectively, and neither disappeared even in the Weimar Republic. 12 13
In Germany the weight of these laws was laid bare by the Eulenburg affair of 1907–1909. From 1906 the conservative journalist Maximilian Harden attacked Philipp zu Eulenburg, Wilhelm II’s intimate, and the Liebenberg circle around him as a “homoerotic round table of political weaklings”, setting off courts martial, five ordinary trials and libel suits. It became the greatest scandal of the German Empire, drew attention around the world and made homosexuality a question of politics and foreign policy. 14
In Britain the Criminal Law Amendment Act of 1885, “to make further provision for the Protection of Women and Girls”, raised the age of consent from 13 to 16 and through its section 11, the Labouchère Amendment, made “gross indecency” between men a crime. In practice the section was used broadly to prosecute homosexuals where sodomy could not be proven; most famously, Oscar Wilde was sentenced under it to two years’ hard labour in 1895. Only in 1967 was it lifted in England and Wales for consenting adults. 15 16 17
In Britain the emblem of the law became the trials of Oscar Wilde in 1895. Wilde sued the Marquess of Queensberry, father of his lover Lord Alfred Douglas, for libel; evidence unearthed at the hearings made him drop the case and led to his own arrest for gross indecency with other men. The jury at the first criminal trial could not reach a verdict; under cross-examination he described “the love that dare not speak its name” as the great affection of an elder for a younger man such as that between David and Jonathan. At the retrial he was sentenced to two years’ hard labour, the maximum penalty; he wrote De Profundis in prison, left for France on the day of his release, wrote The Ballad of Reading Gaol there and died on 30 November 1900. 17
Tsarist Russia punished both. Article 1462 of the Code of Criminal and Correctional Punishments of 1845 made both abortionist and pregnant woman criminals, though it was seldom enforced, and the code in force from 1835 first made “sodomy” a crime for the whole civilian population. The revolution of 1917 swept these laws away, and the Soviet experiment that followed drew the attention of Europe’s sex reformers. 18 19




The high tide of the 1920s: the Soviet experiment, the Institute for Sexual Science and the World League for Sexual Reform
The most radical experiment of the 1920s was Soviet Russia. In November 1920 the Commissariats of Health and Justice made it the first country in the world to legalise abortion, and the criminal codes of 1922 and 1926 did not punish consensual sex between men. In January 1923 a delegation led by Health Commissar Semashko visited the Institute for Sexual Science in Berlin, and Grigory Batkis told the world in The Sexual Revolution in Russia that Soviet law treated sex as a private matter. The event on the Soviet legalisation and ban of abortion and the recriminalisation of sodomy follows the experiment in detail. 20 19 21
The Soviet experiment reached marriage and the family too. The 1918 Code on Marriage, the Family and Guardianship allowed easily obtainable “no-grounds” divorce and abolished “illegitimacy” as a legal concept, entitling every child to parental support. The new code of 1926 extended alimony to unregistered, de facto wives to protect the women left jobless and unprotected under the New Economic Policy. To the jurists the family was a transitional institution that would “wither away” once communal care existed. 22
In Germany Hirschfeld opened the world’s first institute of sexology in Berlin-Tiergarten on 6 July 1919. Besides research on sexual life it served as a counselling centre for people in distress, and it remained the only one of its kind until it was looted on 6 May 1933. Hirschfeld organised a congress for sexual reform in Berlin in 1921, later counted as the first congress of the World League for Sexual Reform, which was formally founded at a congress in Copenhagen in 1928. The event on the German sexual reform movement follows its rise and destruction. 23 24
In the Weimar Republic reform reached the threshold of politics. The film Different from the Others (1919), co-written by Hirschfeld, who also appeared in it, attacked Paragraph 175 and was one of the first sympathetic portrayals of gay men in cinema, though censorship soon restricted it to doctors and researchers. In the Reichstag penal code committee in 1929 the KPD, SPD and DDP struck out by 15 votes to 13 the punishment of simple homosexuality between adult men, and when the physicians Friedrich Wolf and Else Kienle were arrested on 20 February 1931 on charges of performing abortions for profit, a mass movement rose against Paragraph 218. 25 12 13
The World League for Sexual Reform was the thread that tied the national movements together. Its ten-point platform included economic, political and sexual equality of men and women, the secularisation and reform of marriage and divorce law, birth control, “eugenic birth selection”, the protection of unmarried mothers and “illegitimate” children, a rational understanding of intersex people and homosexuals, and comprehensive sex education. Its congresses met in Copenhagen (1928), London (1929), Vienna (1930) and Brno (1932), with speakers such as Hirschfeld, Norman Haire, Dora Russell, Stella Browne, Marie Stopes, Alexandra Kollontai and Max Hodann. By 1930 it claimed 190,000 members through affiliated organisations, among them the Scientific-Humanitarian Committee, the British Society for the Study of Sex Psychology and the League for the Protection of Mothers. A Spanish section followed in 1932 with Gregorio Marañón as president and Hildegart Rodríguez as secretary; it was socially the most conservative and one of the few that left homosexuality out of its programme. 24 26
In Spain sexual reform flowered briefly with the Republic. Hildegart Rodríguez, who at eighteen had become Spain’s youngest lawyer, was a member of the World League and of the Spanish League for Sexual Reform and wrote sixteen books and more than 150 articles; her work on sexology was known to and valued by Hirschfeld, Havelock Ellis, H. G. Wells and Marañón, among others. Vice-president of the PSOE’s youth organisation, she left the party in the Republic’s first year for republicanism and, increasingly critical of Marxism, drew close to anarchism. 27


Socialists and sex: from Kollontai to Reich
After the revolution Soviet socialists argued over a new sexual morality. Alexandra Kollontai advocated free love, but held that casual encounters under continuing inequality between the sexes would leave women exploited and raising children alone. The “glass of water” theory, that sexual desire should be satisfied as simply as drinking a glass of water, was often wrongly attributed to her; according to Clara Zetkin’s recollections Lenin called it “completely un-Marxist and, moreover, anti-social”. Aron Zalkind’s Twelve Sexual Commandments of the Revolutionary Proletariat (1924) demanded abstinence and monogamy and declared that sex must in every respect be subordinate to class. 28 29 30
In Germany it was Wilhelm Reich who brought sexual politics into the workers’ movement. In autumn 1931 he founded Sex-Pol within the KPD, demanding new laws on marriage and abortion, and argued the Freudo-Marxist thesis that capitalist sexual repression produced mass neurosis and authoritarianism. Without influence in either psychoanalysis or the communist movement at the time, the theory became one of the foundations of the New Left and the 1968 movement. 31 32
Reich himself did not regard homosexuality as natural sexuality. He took normal sexual development to be heterosexual and explained homosexuality as the result of an early developmental disturbance, writing that fascism’s ascetic education turned the desire for the other sex into homosexual and sadistic desires. Yet he opposed punishing homosexuals to the end, condemning the Soviet recriminalisation of 1936 as a brake on the sexual revolution. The historian Cat Moir judges that his view of homosexuality as a pathology rather than a crime was not far from Hirschfeld’s. 33
But the left was never of one mind. The German communists and Social Democrats supported repeal of Paragraph 175, yet when the homosexuality of the SA chief Ernst Röhm became public in 1931 they used it in political attacks. Over the Reichstag arsonist Marinus van der Lubbe in 1933 the Comintern used aggressively homophobic rhetoric, which may have influenced the Soviet stance. A current that saw sexual liberation as part of the revolution and another that saw homosexuality as “bourgeois degeneracy” lived side by side in the same movement. 19 11


Eugenics across left and right
The sexual reform movements of the period were deeply entangled with eugenics. The eugenics movement, named by Francis Galton in 1883, arose in Britain and spread to most of Europe, including Sweden and Germany. The World League for Sexual Reform listed “eugenic birth selection” in its platform, and Hirschfeld’s institute too was shaped by the eugenic theses and biologism of its time. Marie Stopes, who led the British birth-control movement, campaigned for eugenics and women’s rights together and dispensed cervical caps called “Pro-Race” at her clinic; when Stöcker in Germany demanded the right to abortion, ideas of “breeding up” and of lives unworthy of life were mixed in. 34 24 23 35 13
In Spain too eugenics was a language of the reformers. Gregorio Marañón, president of the Spanish League for Sexual Reform, echoed the international eugenics current of the 1920s and 1930s by taking part in the First Eugenics Conference of 1933; his eugenics started from the high infant mortality and poor care of children in 1920s Spain. During the Republic he opposed artificial contraception and state sex education, which he held to be the parents’ task, and to avoid conflict with Catholic morality left the decision to education and individual conscience. 36
The same language served left-wing revolution and right-wing dictatorship alike. The Catalan abortion decree of 1936 was called the “eugenic reform of abortion”, and Hildegart Rodríguez was raised according to her mother Aurora’s eugenic and educational ideals. On the other side the Nazis began the forced sterilisation of the “hereditarily diseased” with the law of 14 July 1933, and the Francoist psychiatrist Antonio Vallejo-Nágera, in Eugenics of Hispanidad and the Regeneration of the Race, preached a “positive eugenics” to “multiply the select and let the weak perish”, casting Republicans as mentally inferior. 37 38 13 39
Britain’s different path: gradual reform amid courts and censorship
Britain had neither a revolution nor a fascist regime, but its sexual reform advanced slowly, bound by courts and censorship. After the Wilde trials Edward Carpenter defended homosexuality in The Intermediate Sex (1908) as the natural orientation of a “third sex”, and the book became a foundation of the British movement for gay rights. In 1913 the British Society for the Study of Sex Psychology was founded on the writings of Carpenter and Ellis, with Hirschfeld as a co-founder, to fight legal discrimination against homosexuals; in 1931 it was renamed the British Sexological Society. 11 40 41
Carpenter was a model for the British movement in his life as well as his theory. In 1883 he built a house at Millthorpe in Derbyshire and lived the “simple life”, making and selling sandals, and in 1891 he met George Merrill, a working man from the Sheffield slums, with whom he lived from 1898 for the rest of his life. In The Intermediate Sex he wrote that “Eros is a great leveller”, and that affection crossing the bounds of class and caste might be the firmest basis of true democracy. Combining socialism with the defence of homosexuality, he stood for a British tradition distinct from both the German masculinist current and the medical theory of a “third sex”. 40
In birth control Marie Stopes led the way. Her sex manual Married Love (1918) was controversial and brought birth control into wide public discourse, and on 17 March 1921 she opened Britain’s first birth-control clinic in Holloway, London; publicly, however, she opposed abortion. In 1928 Radclyffe Hall’s lesbian novel The Well of Loneliness, after a campaign by the Sunday Express editor James Douglas, who wrote that he “would rather give a healthy boy or a healthy girl a phial of prussic acid than this novel”, was judged obscene and not published again in England until 1949. 35 42
In the 1930s British reform proceeded by widening gaps in the law. In 1936 Stella Browne and others founded the Abortion Law Reform Association, and in 1938, in Rex v Bourne, the surgeon Aleck Bourne was acquitted for terminating the pregnancy of a 14-year-old girl who had been raped, setting the precedent that abortion might be lawful to protect the mother’s health. The precedent led on to the Abortion Act of 1967. Britain’s story is told in a sister event of its own. 43 44


The reaction of the 1930s: Berlin, Moscow, Madrid
In the 1930s Europe’s sexual reform was crushed under three dictatorships in turn. In May 1933 Germany’s National Socialist regime raided the Institute for Sexual Science, destroying the World League’s books and records too, and on 30 June–2 July 1934, in the Night of the Long Knives, it killed Röhm and the SA leadership. On 1 September 1935 Paragraph 175 was extended from acts resembling intercourse to all “lewd” acts, and Paragraph 175a added for “aggravated cases”. The same year the World League for Sexual Reform ceased to exist. 24 45 12
The National Socialist regime’s abortion policy was likewise split in two by racism. For the “hereditarily diseased” a 1935 amendment allowed abortion together with sterilisation, while for the “German people” abortion that “continuously impaired the vitality of the German people” carried the death penalty from 1943. The “voluntary and responsible birth” the reformers had sought in the name of eugenics was thus turned into a system in which the state decided who should and should not bear children. 13
The Soviet Union changed course at the same time. The World League’s fifth congress was to have met in Moscow, but the plans were dropped amid the political tensions of Hitler’s and Stalin’s growing power. After the OGPU arrested several hundred homosexuals in 1933, sodomy returned to the RSFSR criminal code in 1934, and a decree of 27 June 1936 banned abortion without medical grounds. The abortion ban lasted until 1955, the sodomy law until 1993. 24 19 20
In Spain the Franco regime, victorious in the civil war, rolled back the Republic’s sexual reform. The divorce law of 1932 was suspended in the rebel zone on 2 March 1938 and explicitly repealed on 23 September 1939, and a decree of 6 November 1941 refounded the Patronato de Protección a la Mujer for the “moral dignity” of young women. The Sección Femenina, founded in Madrid in 1934, became under Pilar Primo de Rivera an almost monopolistic women’s organisation spreading a Catholic ideal of womanhood. The event on sexual reform in the Second Spanish Republic follows the Spanish story. 46 47 48
The reaction also broke the lives of the movement’s leaders. Warned off, Hirschfeld never set foot in Germany again after 1932, living in exile in Zurich, Ascona, Paris and finally Nice, where he died on his 67th birthday, 14 May 1935. Hildegart Rodríguez, secretary of the Spanish League for Sexual Reform, was killed in 1933, at the height of the Republic, by the mother who had tried to raise her as “the woman of the future”. The German movement’s people went into exile or took their lives, and most of the Spanish reformers took the road of exile in 1939. 49 38



Assessment: diverging fates in the same years
Europe’s sexual reform movements went furthest in the 1920s, crossing borders and taking one another as reference, and collapsed at the same pace in the 1930s. Hirschfeld’s institute and the World League linked activists in Germany, the Soviet Union, Britain and Spain, and the Soviet legalisation of abortion became a point of reference for Western campaigners. Yet the movement was divided within over homosexuality, abortion and eugenics, and the support of left parties wavered with the political moment. 24 23 20
The traces left by the reaction lasted differently in each country. Germany’s Paragraph 175 was eased first in the East, while in the West the Nazi-era text lasted until 1969, and the article was repealed after reunification in 1994. The Soviet sodomy law lived on as Article 121 of the 1960 code until 1993. In Spain homosexuality was removed from the social danger law in 1978, and divorce was recognised again in 1981. Britain, without revolution or dictatorship, took the slowest but unbroken path, through the Bourne judgment of 1938 and the Sexual Offences and Abortion Acts of 1967. The new sexual revolution of the 1960s called up theorists of the 1930s such as Reich again. 12 19 50 46 16 44 32
The World League’s demands of 1928, the secularisation of marriage and divorce law, birth control, protection of unmarried mothers and “illegitimate” children, a rational understanding of homosexuals and the legalisation of sexual acts between consenting adults, were mostly realised only half a century later. The League asked that sexual abnormalities be treated medically rather than “as crimes, vices or sins”, a demand that freed people from punishment but also left behind the view of homosexuality as an illness. 24
Views of these movements changed too. In the first volume of The History of Sexuality (1976) Michel Foucault questioned the “repressive hypothesis” itself, the belief that sexuality was repressed from the seventeenth to the early twentieth century before a partial liberation in modern times. Sexuality, he argued, had never truly been repressed, and portraying the past as repressed had provided the basis for the belief that future sexuality could be free. The “liberation” the reformers of the 1920s and 1930s dreamed of came to be reread in that critical light. 51
Consequences
In 1933–1935 the National Socialist regime looted the Institute for Sexual Science and tightened Paragraph 175, and the World League ceased to exist in 1935. The Soviet Union restored its sodomy law in 1934 and banned abortion in 1936, and the Franco regime repealed the divorce law in 1939 and set up the Patronato in 1941. Only Britain, without revolution or dictatorship, kept to a path of gradual reform, through the Bourne judgment of 1938 to the Sexual Offences and Abortion Acts of 1967.
Timeline & Map
- 11867.08.29Ulrichs speaks out
Ulrichs defended homosexuality in public at the congress of German jurists in Munich.
- 21877Bradlaugh–Besant trial
The two were prosecuted for publishing Knowlton’s birth-control book.
- 31885Labouchère Amendment
Section 11 of the Criminal Law Amendment Act made “gross indecency” between men a crime.
- 41895Wilde convicted
Oscar Wilde was sentenced to two years’ hard labour under the Labouchère Amendment.
- 51897.05.15Scientific-Humanitarian Committee founded
Hirschfeld and others founded the first gay rights organisation in Berlin.
- 61898.01.13Bebel’s petition against Paragraph 175
Bebel openly supported homosexual emancipation in the Reichstag.
- 71919.07.06Institute for Sexual Science opened
The world’s first institute of sexology opened in Berlin-Tiergarten.
- 81920.11Soviet Russia legalises abortion
The Commissariats of Health and Justice legalised abortion, the first in the world.
- 91921.03.17Stopes’s clinic
Marie Stopes opened Britain’s first birth-control clinic in London.
- 101928World League for Sexual Reform founded
The League was formally founded at the Copenhagen congress.
- 111928The Well of Loneliness judged obscene
A British court judged Radclyffe Hall’s novel obscene.
- 121932Spanish League for Sexual Reform
The League’s Spanish section was founded with Marañón as president.
- 131933.05.06Institute for Sexual Science looted
Nazi students looted the institute, destroying the League’s records too.
- 141934.04.01Soviet sodomy law
Article 154-a punishing sex between men entered the RSFSR criminal code.
- 151935.09.01Paragraph 175 tightened
The Nazis extended Paragraph 175 to all “lewd” acts; the League ceased to exist that year.
- 161936.06.27Soviet abortion ban
Abortion without medical grounds was banned.
- 171936.12.25Catalonia legalises abortion
The Generalitat adopted the decree on the “eugenic reform of abortion”.
- 181938Rex v Bourne
A surgeon who terminated a raped girl’s pregnancy was acquitted.
- 191939.09.23Spanish divorce law repealed
The Franco regime explicitly repealed the divorce law of 1932.
- 201941.11.06Patronato refounded
The Franco regime refounded the Patronato de Protección a la Mujer.
Related people 29
Leadership3
Founded the Scientific-Humanitarian Committee and the Institute for Sexual Science and linked national movements through the World League.
British sexologistHavelock Ellis1859–1939His Sexual Inversion was the first objective study of homosexuality as neither disease nor crime.
Leader of the League for the Protection of MothersHelene Stöcker1869–1943Called for the protection of unmarried mothers, sex education, contraception and the repeal of Paragraph 218.
Implementers & commanders1
Participants16
Wrote Woman and Socialism and brought the petition against Paragraph 175 before the Reichstag in 1898.
Author of The Origin of the FamilyFriedrich Engels1820–1895Wrote the founding socialist account tying the monogamous family to private property.
Neo-Malthusian campaignerAnnie Besant1847–1933Was prosecuted with Bradlaugh in 1877 for publishing Knowlton’s birth-control book.
Socialist writerEdward Carpenter1844–1929Laid a foundation of the British gay rights movement with The Intermediate Sex.
Birth-control campaignerMarie Stopes1880–1958Wrote Married Love and opened Britain’s first birth-control clinic in 1921.
Socialist feministStella Browne1880–1955Spoke at World League congresses and co-founded the Abortion Law Reform Association in 1936.
Co-president of the World LeagueNorman Haire1892–1952Led the London congress of 1929.
British Labour activistDora Russell1894–1986Spoke at World League congresses.
Soviet Zhenotdel leaderAlexandra Kollontai1872–1952Advocated free love and was among the World League’s congress speakers.
Soviet Health CommissarNikolai Semashko1874–1949Led the legalisation of abortion in 1920 and visited the Institute for Sexual Science in 1923.
Social hygienistGrigory Batkis1895–1960Told the world about Soviet sexual legislation in The Sexual Revolution in Russia.
PsychiatristAron Zalkind1887–1936Wrote the Twelve Sexual Commandments of the Revolutionary Proletariat.
Founder of Sex-PolWilhelm Reich1897–1957Founded Sex-Pol inside the KPD and argued Freudo-Marxism.
President of the Spanish League for Sexual ReformGregorio Marañón1887–1960Became president of the World League’s Spanish section in 1932.
Secretary of the Spanish League for Sexual ReformHildegart Rodríguez Carballeira1914–1933Raised according to her mother’s eugenic ideals, she led the Spanish sex-reform movement.
Catalan health undersecretaryFélix Martí Ibáñez1911–1972Drove the Catalan abortion decree of 1936.
Opposition & resistance4
The sodomy law of 1934 and the abortion ban of 1936 came under his leadership.
Chancellor of GermanyAdolf Hitler1889–1945Ordered the killing of Röhm and the SA leadership in the Night of the Long Knives.
Spanish head of stateFrancisco Franco1892–1975Repealed the divorce law and rolled back the Republic’s sexual reform.
Francoist psychiatristAntonio Vallejo-Nájera1888–1960Preached “positive eugenics” and cast Republicans as inferior.
Targets & victims3
Sources: reference 1reference 2reference 3reference 4reference 5reference 6reference 7reference 8reference 9reference 10reference 11reference 12reference 13reference 14reference 15reference 16reference 17reference 18reference 19reference 20reference 21reference 22reference 23reference 24reference 25reference 26reference 27reference 28reference 29reference 30reference 31reference 32reference 33reference 34reference 35reference 36reference 37reference 38reference 39reference 40reference 41reference 42reference 43reference 44reference 45reference 46reference 47reference 48reference 49reference 50reference 51