Twin

Fraternal twin boys in the tub

The term twin most notably refers to two individuals (or one of two individuals) who have shared the same uterus (womb) and are usually, but not necessarily, born on the same day. A fetus alone in the womb is called a singleton. Due to the limited size of the mother's womb, multiple pregnancy is much less likely to carry to full term than singleton birth (twins usually around 34 to 36 weeks). Since some premature births often have health consequence to the babies, twins birth are often handled with special procedures.

Types of twins

Fraternal twins

Fraternal twins (commonly known as "non-identical twins") usually occur when two fertilized eggs are implanted in the uterine wall at the same time. The two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as "biovular" twins.

Dizygotic twins, like any siblings, have a very small chance of having the exact same chromosome profile, but most likely have a number of different chromosomes that distinguish them. Dizygotic twins may be a different sex or the same sex, just as with any other siblings.

Studies show that there is a genetic basis for fraternal twinning—that is, non-identical twins do run in families. However, it is only the female that has any influence on the chances of having fraternal twins as the male cannot make her release more than one ovum.

Identical twins

Identical twins occur when a single egg is fertilized to form one zygote (monozygotic) but the zygote then divides into two separate embryos. The two embryos develop into fetuses sharing the same womb. Depending on the stage at which the zygote divides, identical twins may share the same amnion (in which case they are known as monoamniotic) or not (diamniotic). Diamniotic identical twins may share the same placenta (known as monochorionic) or not (dichorionic). All monoamniotic twins are monochorionic. Also note that any monochorionic or monoamniotic twins are identical twins. This condition does not occur for fraternal twins.

The later in pregnancy that twinning occurs, the more structures will be shared. Zygotes that twin at the earliest stages will be diamniotic and dichorionic ("di-di"). Twinning between 4 to 8 days after fertilization typically results in monochorionic-diamniotic ("mono-di") twins. Twinning between 8 to 12 days after fertilization will usually result in monochorionic-monoamniotic ("mono-mono") twins. Twinning after 12 days post-fertilization will typically result in conjoined twins.

Sharing the same amnion (or the same amnion and placenta) can cause complications in pregnancy. For example, the umbilical cords of monoamniotic twins can become entangled, reducing or interrupting the blood supply to the developing fetus. Monochorionic twins, sharing one placenta, usually also share the placental blood supply. These twins may develop such that blood passes disproportionately from one twin to the other through connecting blood vessels within their shared placenta, leading to twin-to-twin transfusion syndrome. About 50% of mono-mono twins die from umbilical cord entanglement.

Monozygotic twins are genetically identical unless there has been a mutation in development, and they are almost always the same gender. (On extremely rare occasions, an original XXY zygote may form monozygotic boy/girl twins by dropping the Y chromosome for one twin and the extra X chromosome for the other.) Monozygotic twins generally look alike, although sometimes they appear as mirror images of each other. Examination of details such as fingerprints can tell them apart. As they mature, identical twins often become less alike because of lifestyle choices or external influences such as scars.

While it was originally thought that identical twins do not run in families, but occur more or less randomly, some recent research has suggested that a genetic predisposition may exist. The exact cause for the splitting of a zygote or embryo is unknown.

Identical twins can behave as differently as any other siblings (a matter of much interest to psychologists). They develop their own individual personalities to enable themselves to be identified as individual persons. Many identical twins spend most of their time together (especially as children), so people often assume that they will behave alike just as they look alike; however, this is not the case. Twins are unique individuals that establish their own individual likes and dislikes. There are usually obvious signs of differences when the identical twins are observed separately or together.

Identical twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off. This is called epigenetic modification. A study of 80 pairs of twins ranging in age from 3 to 74 showed that the youngest twins have relatively few epigenetic differences. The number of differences between identical twins increases with age. 50-year-old twins had over 3 times the epigenetic difference that the 3-year-old twins had. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. (Fraga, et al., 2005).

Some percentage of monozygotic twins are called "mirror twins" or mirror image twins. These are identical twins with opposite features, that is one may be right handed and the other may be left handed; hair will whorl in the opposite direction, and so on. The incidence of mirror twinning is comparatively rare. They result from a late split of the fertilized egg at around 9-12 days. One mirror may or may not have situs inversus. This is where some or all of the organs will be on the opposite side of the body, such as the heart being on the right(Dextrocardia). Such conditions are usually associated with a higher incidence of other birth defects.

Complications of twin pregnancy

Vanishing twins

Researchers suspect that more pregnancies start out as multiples than come to term that way. Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes.

Conjoined twins

Conjoined twins are monozygotic twins, whose bodies are joined together at birth. This occurs where the single zygote of identical twins fails to separate completely. This condition occurs in about 1 in 100,000 pregnancies.

Parasitic twins

Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. One fetus acts as a parasite towards the other.

Sometimes the parasitic twin just becomes an almost indistinguishable part of the other.

A chimera is a person who is a completely normal human with no extra parts, but some of the parts actually came from his or her twin. A chimera may arise either from identical twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which could be identified by chromosomal comparisons from various parts of the body.

Miscarried twin

Occasionally, a woman will suffer a miscarriage early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. Similar to vanishing twin.

Human twins

Historically, about 1 in 80 human births (1.2%) has been the result of a twin pregnancy. The rate of twinning varies greatly among ethnic groups, ranging as high as about 6% for the Yoruba or 10% for a tiny Brazilian village (see [1]). The widespread use of fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an "epidemic of multiple births". In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. Thus, approximately 6% of children born in the US in 2001 were twins.

Nevertheless, the rate of identical twins remains at about 1 in 250 across the globe, further suggesting that pregnancies resulting in identical twins occur randomly.

Multiple births

Sometimes multiple births may involve more than two fetuses. If there are three, they are called triplets; four, quadruplets; five, quintuplets; six, sextuplets, seven, septuplets, and so on. Before the advent of ovulation-stimulating drugs, triplets were quite rare (approximately 1 in 8000 births) and higher order births so rare as to be almost unheard of. Multiple pregnancies are usually delivered before the full term of 40 weeks gestation: the average length of pregnancy is around 36 weeks for twins, 34 weeks for triplets and 32 weeks for quadruplets.

Predisposing factors

The cause of monozygotic twinning is unknown. Fewer than 20 families have been described with an inherited tendency towards monozygotic twinning (people in these families have nearly a 50% chance of delivering monozygotic twins). Some evidence suggests that the environment of the womb causes the zygote to split in most cases.

Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:

  • She is of African descent
  • Between the age of 30 and 40 years
  • Greater than average height and weight
  • Several previous pregnancies.

Women undergoing certain fertility treatments may have a greater chance of multiple births. This can vary depending on what types of fertility treatments are used. With in vitro fertilisation (IVF), this is primarily due to the insertion of multiple embryos into the uterus. Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, allowing the possibility of multiples. Many fertility treatments have no effect on the likelihood of multiple births.

Twin studies

Twin studies are studies that assess identical (monozygotic) twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from environmental influence. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been invaluable in the exploration of human nature.

Unusual Twinnings

There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are so unusual that most obstetricians or midwives may go their entire careers without encountering a single case.

Among fraternal twins, in rare cases, the eggs are fertilised at different times with two or more acts of sexual intercourse, either within one menstrual cycle (superfecundation) or, even more rarely, later on in the pregnancy (superfetation). This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings). One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins whose parents were involved in paternity suits was approximately 2.4%; see the references section, below, for more details.

Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). The probability of this is so vanishingly small (only 3 documented cases) that multiples having different genders is universally accepted as a sound basis for a clinical determination that in utero multiples are not monozygotic. When monozygotic twins are born with different genders it is because of chromosomal birth defects. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.

Twinning in animals

Multiple births are common in many animal species, such as cats, sheep, and ferrets. The incidence of twinning among cattle is about 1-4%, and research is underway to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed.


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The incidence of twinning among cattle is about 1-4%, and research is underway to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed. These include:. Multiple births are common in many animal species, such as cats, sheep, and ferrets. This in turn means that soldiers have to be trained to fight in a specific type of terrain. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes. The terrain over which a war is fought has a big impact on the type of combat which takes place. When monozygotic twins are born with different genders it is because of chromosomal birth defects. Non-lethal chemical weapons, such as tear gas and pepper spray, are widely used.

The probability of this is so vanishingly small (only 3 documented cases) that multiples having different genders is universally accepted as a sound basis for a clinical determination that in utero multiples are not monozygotic. Various treaties have sought to ban its further use. Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). Poison gas as a chemical weapon was principally used during World War I, and resulted in an estimated 91,198 deaths and 1,205,655 injuries. One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins whose parents were involved in paternity suits was approximately 2.4%; see the references section, below, for more details. Intentional air pollution in combat is called chemical warfare. This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings). Military action produces a very small percentage of air pollution emissions.

Among fraternal twins, in rare cases, the eggs are fertilised at different times with two or more acts of sexual intercourse, either within one menstrual cycle (superfecundation) or, even more rarely, later on in the pregnancy (superfetation). Terrorism can be considered an extreme form of asymmetrical warfare. There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are so unusual that most obstetricians or midwives may go their entire careers without encountering a single case. The Israeli-Palestinian conflict is a common example of asymmetrical warfare. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been invaluable in the exploration of human nature. This type of war often results in guerrilla tactics. Twin studies are studies that assess identical (monozygotic) twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from environmental influence. Asymmetrical warfare is a conflict between two populations of drastically different levels of military mechanization.

Many fertility treatments have no effect on the likelihood of multiple births. A war where the forces in conflict belong to the same country or empire or other political entity is known as a civil war. Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, allowing the possibility of multiples. (Compare with unconventional warfare and nuclear warfare.). With in vitro fertilisation (IVF), this is primarily due to the insertion of multiple embryos into the uterus. "Conventional warfare" describes either:. This can vary depending on what types of fertility treatments are used. This usage is not always recognized as valid, however, particularly by those who do not accept the connotations of the term.

Women undergoing certain fertility treatments may have a greater chance of multiple births. When one country sends armed forces to another, allegedly to restore order or prevent genocide or other crimes against humanity, or to support a legally recognized government against insurgency, that country sometimes refers to it as a police action. Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:. Smaller armed conflicts are often called riots, rebellions, coups, etc. Some evidence suggests that the environment of the womb causes the zygote to split in most cases. Wars are a natural outgrowth of the free market and class system, and will not disappear until a world revolution occurs. Fewer than 20 families have been described with an inherited tendency towards monozygotic twinning (people in these families have nearly a 50% chance of delivering monozygotic twins). It sees wars as imperial ventures to enhance the power of the ruling class and divide the proletariat of the world by pitting them against each other for contrived ideals such as nationalism or religion.

The cause of monozygotic twinning is unknown. The economic theories also form a part of the Marxist theory of war, which argues that all war grows out of the class war. Multiple pregnancies are usually delivered before the full term of 40 weeks gestation: the average length of pregnancy is around 36 weeks for twins, 34 weeks for triplets and 32 weeks for quadruplets. It is most often advocated by those to the left of the political spectrum, who argue that such wars serve the interests of the wealthy, but are fought by the poor. Before the advent of ovulation-stimulating drugs, triplets were quite rare (approximately 1 in 8000 births) and higher order births so rare as to be almost unheard of. invasion of Iraq. If there are three, they are called triplets; four, quadruplets; five, quintuplets; six, sextuplets, seven, septuplets, and so on. Unquestionably a cause of some wars, from the empire building of Britain to the 1941 Nazi invasion of the Soviet Union in pursuit of oil, this theory has been applied to many other conflicts including the 2003 U.S.

Sometimes multiple births may involve more than two fetuses. In this view, wars begin as a pursuit of new markets, of natural resources, and of wealth. Nevertheless, the rate of identical twins remains at about 1 in 250 across the globe, further suggesting that pregnancies resulting in identical twins occur randomly. Another school of thought argues that war can be seen as an outgrowth of economic competition in a chaotic and competitive international system. Thus, approximately 6% of children born in the US in 2001 were twins. For example, Sweden made efforts to deceive Nazi Germany that it would resist an attack fiercely partly by playing on the myth of Aryan superiority, and by making sure that Hermann Göring only saw Elite troops in action, often dressed up as regular soldiers, when he came to visit. In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. If you think that you can convince the opponent that you will fight, the opponent might desist.

The widespread use of fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an "epidemic of multiple births". One major difficulty is that in a conflict of interests, some deception or at least not telling everything is a standard tactical component on both sides. The rate of twinning varies greatly among ethnic groups, ranging as high as about 6% for the Yoruba or 10% for a tiny Brazilian village (see [1]). The American decision to enter the Vietnam War was made with the full knowledge that the communist forces would resist them, but did not believe that the guerrillas had the capability to long oppose American forces. Historically, about 1 in 80 human births (1.2%) has been the result of a twin pregnancy. The Argentinean dictatorship knew that the United Kingdom had the ability to defeat them, but their intelligence failed them on the question of whether the British would use their power to resist the annexation of the Falklands. Similar to vanishing twin. In theory to have enough information to prevent all wars both need to be fully known.

Occasionally, a woman will suffer a miscarriage early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. The first is to find out the ability of an enemy, the second their intent. A chimera may arise either from identical twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which could be identified by chromosomal comparisons from various parts of the body. There are two main objectives in the gathering of intelligence. A chimera is a person who is a completely normal human with no extra parts, but some of the parts actually came from his or her twin. While purely random events, such as storms or the right person dying at the right time, might have had some effect on history, these only influence a single battle or slightly alter the outcome of a war, but would not mean the difference between victory and defeat. Sometimes the parasitic twin just becomes an almost indistinguishable part of the other. This theory is predicated on the notion that the outcome of wars is not randomly determined, but fully determined on factors such as doctrine, economies, and power.

One fetus acts as a parasite towards the other. If it had been known for certainty that the Third Reich would collapse after only a few years of war, the Nazis would not have launched the invasion at all. Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. If in 1940 it had been known with certainty the Germans would dominate central Europe for many decades, it is unlikely the Norwegians would have resisted. This condition occurs in about 1 in 100,000 pregnancies. The Norwegians did not know whether the German domination would be permanent and also felt that noble resistance would win them favour with the Allies and a position at the peace settlement in the event of an Allied victory. This occurs where the single zygote of identical twins fails to separate completely. The Norwegian decision to resist the Nazi invasion was taken with the certain knowledge that Norway would fall.

Conjoined twins are monozygotic twins, whose bodies are joined together at birth. Lack of information may not only be to who wins in the immediate future. Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. The leaders of these nations chose not to resist as they saw the potential benefits being not worth the loss of life and destruction such resistance would cause. Researchers suspect that more pregnancies start out as multiples than come to term that way. On the other hand, Finland's decision to resist a similar Soviet aggression in 1939 led to the Winter War. Such conditions are usually associated with a higher incidence of other birth defects. However, throughout history there are as many invasions and annexations that did not lead to a war, such as the U.S.-led invasion of Haiti in 1994, the Nazi invasions of Austria and Czechoslovakia preceding the Second World War, and the annexation of the Baltic states by the Soviet Union in 1940.

This is where some or all of the organs will be on the opposite side of the body, such as the heart being on the right(Dextrocardia). This notion is made harder to accept because it is far more common to study the cause of wars rather than events that failed to cause wars, and wars are far more memorable. One mirror may or may not have situs inversus. This notion is generally agreed to by almost all scholars of war since Clausewitz. They result from a late split of the fertilized egg at around 9-12 days. This is based on the notion that wars are reciprocal, that all wars require both a decision to attack and also a decision to resist attack. The incidence of mirror twinning is comparatively rare. If both sides at the outset knew the result neither would fight, the loser would merely surrender and avoid the cost in lives and infrastructure that a war would cause.

These are identical twins with opposite features, that is one may be right handed and the other may be left handed; hair will whorl in the opposite direction, and so on. This theory, advanced by scholars of international relations such as Geoffrey Blainey, argues that all wars are based on a lack of information. Some percentage of monozygotic twins are called "mirror twins" or mirror image twins. A popular new approach is to look at the role of information in the outbreak of wars. (Fraga, et al., 2005). This theory accounts for the relative decrease in wars during the past fifty years, especially in the developed world, where advances in agriculture have made it possible to support a much larger population that was formerly the case, and where birth control has dramatically slowed the increase in population. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. Thomas Malthus (1766 - 1834) wrote that populations always increase until they are limited by war, disease, or famine.

50-year-old twins had over 3 times the epigenetic difference that the 3-year-old twins had. This is one of the earliest expressions of what has come to be called the Malthusian theory of war, in which wars are caused by expanding populations and limited resources. The number of differences between identical twins increases with age. Enter upon the road to the Holy Sepulcher; wrest that land from a wicked race, and subject it to yourselves.". A study of 80 pairs of twins ranging in age from 3 to 74 showed that the youngest twins have relatively few epigenetic differences. Let hatred, therefore, depart from among you; let your quarrels end. This is called epigenetic modification. Hence it is that you murder and devour one another, that you wage wars, and that many among you perish in civil strife.

Identical twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off. Pope Urban in 1095, on the eve of the First Crusade, wrote, "For this land which you now inhabit, shut in on all sides by the sea and the mountain peaks, is too narrow for your large population; it scarcely furnishes food enough for its cultivators. There are usually obvious signs of differences when the identical twins are observed separately or together. This differs from the traditional approach of Carl von Clausewitz and Leopold von Ranke that argue it is the decisions of statesmen and the geopolitical situation that leads to war. Twins are unique individuals that establish their own individual likes and dislikes. Thus World War I was not a product of international disputes, secret treaties, or the balance of power but a product of the economic, social, and political situation within each of the states involved. Many identical twins spend most of their time together (especially as children), so people often assume that they will behave alike just as they look alike; however, this is not the case. One based on the works of Eckart Kehr and Hans-Ulrich Wehler sees war as the product of domestic conditions, with only the target of aggression being determined by international realities.

They develop their own individual personalities to enable themselves to be identified as individual persons. Sociology has thus divided into a number of schools. Identical twins can behave as differently as any other siblings (a matter of much interest to psychologists). Rummel has found that civil wars and foreign wars are very different in origin, but Jonathan Wilkenfield using different data found just the opposite. The exact cause for the splitting of a zygote or embryo is unknown. Data looked at by R.J. While it was originally thought that identical twins do not run in families, but occur more or less randomly, some recent research has suggested that a genetic predisposition may exist. Many sociologists have attempted to divide wars into types to get better correlations, but this has also produced mixed results.

As they mature, identical twins often become less alike because of lifestyle choices or external influences such as scars. One correlation that has found much support is that states that are democracies do not go to war with each other, an idea known as the democratic peace theory. Examination of details such as fingerprints can tell them apart. A detailed study by Michael Haas found that no single variable has a strong correlation to the occurrence of wars. (On extremely rare occasions, an original XXY zygote may form monozygotic boy/girl twins by dropping the Y chromosome for one twin and the extra X chromosome for the other.) Monozygotic twins generally look alike, although sometimes they appear as mirror images of each other. So far none of these formulas have successfully predicted the outbreak of future conflicts. Monozygotic twins are genetically identical unless there has been a mutation in development, and they are almost always the same gender. More recent databases of wars and armed conflict have been assembled by the Correlates of War Project, Peter Brecke and the Uppsala Department of Peace and Conflict Research.

About 50% of mono-mono twins die from umbilical cord entanglement. The statistical analysis of war was pioneered by Lewis Fry Richardson following World War I. These twins may develop such that blood passes disproportionately from one twin to the other through connecting blood vessels within their shared placenta, leading to twin-to-twin transfusion syndrome. Some use detailed formulas taking into account hundreds of demographic and economic values to predict when and where wars will break out. Monochorionic twins, sharing one placenta, usually also share the placental blood supply. Sociology has long been very concerned with the origins of war, and many thousands of theories have been advanced, many of them contradictory. For example, the umbilical cords of monoamniotic twins can become entangled, reducing or interrupting the blood supply to the developing fetus. Theorists such as Ashley Montagu emphasize the top down nature of war, that almost all wars are begun not by popular pressure but by the whims of leaders and that these leaders also work to maintain a system of ideological justifications for war.

Sharing the same amnion (or the same amnion and placenta) can cause complications in pregnancy. They see the fighting of animals, the skirmishes of hunter-gatherer tribes, and the organized warfare of modern societies as distinct phenomena each with their own causes. Twinning after 12 days post-fertilization will typically result in conjoined twins. Many anthropologists also see no links between various forms of violence. Twinning between 8 to 12 days after fertilization will usually result in monochorionic-monoamniotic ("mono-mono") twins. To this school the acceptance of war is inculcated into each of us by the religious, ideological, and nationalistic surroundings in which we live. Twinning between 4 to 8 days after fertilization typically results in monochorionic-diamniotic ("mono-di") twins. Thus if human societies could be reformed, war would disappear.

Zygotes that twin at the earliest stages will be diamniotic and dichorionic ("di-di"). They see it as fundamentally cultural, learned by nurture rather than nature. The later in pregnancy that twinning occurs, the more structures will be shared. Several anthropologists take a very different view of war. This condition does not occur for fraternal twins. By this theory, war is another 'opiate of the masses' by which a state controls its people and prevents revolution. Also note that any monochorionic or monoamniotic twins are identical twins. Thus the people are prevented from seeing that their true enemy is in fact their own repressive government.

All monoamniotic twins are monochorionic. War inspires fear and hate among the people of a nation, and gives them a 'legitimate' enemy upon whom they can focus this fear and hate. Diamniotic identical twins may share the same placenta (known as monochorionic) or not (dichorionic). In his fictional book Nineteen-Eighty-Four, George Orwell talks about war being used as one of many ways to distract people. Depending on the stage at which the zygote divides, identical twins may share the same amnion (in which case they are known as monoamniotic) or not (diamniotic). Kennedy, who argue that the organized, sustained war of humans differs more than just technologically from the territorial fights between animals. The two embryos develop into fetuses sharing the same womb. These theories have been criticized by scholars such as John G.

Identical twins occur when a single egg is fertilized to form one zygote (monozygotic) but the zygote then divides into two separate embryos. The earliest advocate of this theory was Konrad Lorenz. However, it is only the female that has any influence on the chances of having fraternal twins as the male cannot make her release more than one ovum. We have the same instincts of a chimpanzee but overwhelmingly more power. Studies show that there is a genetic basis for fraternal twinning—that is, non-identical twins do run in families. However, while war has a natural cause, the development of technology has accelerated human destructiveness to a level that is irrational and damaging to the species. Dizygotic twins may be a different sex or the same sex, just as with any other siblings. This school tends to see war as an extension of animal behaviour, such as territoriality and competition.

Dizygotic twins, like any siblings, have a very small chance of having the exact same chromosome profile, but most likely have a number of different chromosomes that distinguish them. A distinct branch of the psychological theories of war are the arguments based on evolutionary psychology. The two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as "biovular" twins. This extreme school of thought argues leaders that seek war such as Napoleon, Hitler, Bush and Stalin were mentally abnormal. Fraternal twins (commonly known as "non-identical twins") usually occur when two fertilized eggs are implanted in the uterine wall at the same time. Other psychologists have argued that while human temperament allows wars to occur, they only do so when mentally unbalanced men are in control of a nation. . Critics, of course, point to various examples of female political leaders who had no qualms about using military force, such as Margaret Thatcher or Indira Gandhi.

Since some premature births often have health consequence to the babies, twins birth are often handled with special procedures. This theory has played an important role in modern feminism. Due to the limited size of the mother's womb, multiple pregnancy is much less likely to carry to full term than singleton birth (twins usually around 34 to 36 weeks). One alternative is to argue that war is only, or almost only, a male activity and if human leadership was in female hands wars would not occur. A fetus alone in the womb is called a singleton. If war is innate to human nature, as is presupposed by many psychological theories, then there is little hope of ever escaping it. The term twin most notably refers to two individuals (or one of two individuals) who have shared the same uterus (womb) and are usually, but not necessarily, born on the same day. Periods that are seen as peaceful are actually periods of preparation for a later war or when war is suppressed by a state of great power, such as the Pax Britannica.

Several previous pregnancies. A solution adapted to this problem by militarists such as Franz Alexander is that peace does not really exist. Greater than average height and weight. If the innate psychology of the human mind is unchanging, these variations are inconsistent. Between the age of 30 and 40 years. In addition, they raise the question why there are sometimes long periods of peace and other eras of unending war. She is of African descent. While these theories may have some explanatory value about why wars occur, they do not explain when or how they occur.

This combines with other notions, such as displacement where a person transfers their grievances into bias and hatred against other ethnic groups, nations, or ideologies. While this violence is repressed in normal society it needs the occasional outlet provided by war. Durban and John Bowlby have argued that human beings, especially men, are inherently violent. Psychologists such as E.F.M.

Social scientists criticize this approach arguing that at the beginning of every war some leader makes a conscious decision and that they cannot be seen as purely accidental. There are some conditions and situations that make them more likely but there can be no system for predicting where and when each one will occur. Taylor famously described wars as being like traffic accidents. P.

J. A. Historians tend to be reluctant to look for sweeping explanations for all wars. Representatives of many different academic disciplines have attempted to explain war.

There is great debate over why wars happen, even when most people do not want them to. Sometimes the term "war" will not be used in order to circumvent national constitutions which restrict the power of the executive to wage war without the agreement of other branches of government. For example, the United States Government referred to the Korean War as a "police action", and the British Government was very careful to use the term "armed conflict" instead of "war" during the Falklands War in 1982 to comply with the letter of international law. This has resulted in wars (in the sense defined in the introduction to this article) without formal declaration and combatants who officially choose terms other than "war," such as:.

Sometimes the term "war" is restricted by legal definition to those conflicts where one or both belligerents have formally declared war. By only illegalising "war against the rules", it is alleged, such treaties and conventions, in effect, sanction certain types of war. It must be noted that in war such treaties are generally thrown to one side if they interfere with the vital interests of either side; some have criticised such conventions as simply providing a fig leaf for the inhuman practice of war. 135, entered into force 21 October 1950.

A couple of examples are: Resolutions of the Geneva International Conference, Geneva, 26 October-29 October 1863 and Geneva Convention relative to the Treatment of Prisoners of War, 75 U.N.T.S. Treaty signing has since been a part of international diplomacy, and too many treaties to mention in this scant article have been signed. The most pervasive of those are the Geneva Conventions, the earliest of which began to take effect in the mid 1800s. A number of treaties regulate warfare, collectively referred to as the laws of war.

Charter, "save succeeding generations from the scourge of war.". The United Nations is the latest and most comprehensive attempt to, as stated in the preamble of the U.N. In modern times, increasing international attention has been paid to peacefully resolving conflicts which lead to war. In some cultures, for example, conflicts have been highly ritualized to limit actual loss of life.

While culture, law, and religion have all been factors in causing wars, they have also acted as restraints at times. Total war is the modern term for the targeting of civilians and the mobilization of an entire society; when every member of the society has to contribute to the war effort. Limitations on the targeting of civilians, what type of weapons can be used, and when combat is allowed have all fallen under these rules in different conflicts. At times throughout history, societies have attempted to limit the cost of war by formalizing it in some way.

Today, some see only just wars as legitimate, and believe that it is the goal of organizations such as the United Nations to unite the world against wars of unjust aggression. The defeat and repudiation of the fascist states and their militarism in the Second World War, the huge psychological and physical damage of nuclear war and a growth of the respect for the sanctity of individual life, as enshrined in the concept of human rights and as a cultural consequence of falling natural mortality rates and birth rates, have contributed to the current view of war. At the outbreak of World War I the writer Thomas Mann wrote, "Is not peace an element of civil corruption and war a purification, a liberation, an enormous hope?" This attitude was embraced by many societies from Sparta in Ancient Greece and the Ancient Romans to the fascist states of the 1930s. Many thinkers, such as Heinrich von Treitschke saw war as humanity's highest activity where courage, honor, and ability were more necessary than in any other endeavour.

The negative view of war has not always been held as widely as it is today. Gandhi (called "Mahatma" or "Great Soul"). This position was passionately defended by the Indian leader Mohandas K. Pacifists believe that war is inherently immoral and that no war should ever be fought.

Today war is generally seen as undesirable and morally problematic, although this view is contested by some. Although many ancient nations and some more modern ones viewed war as noble, over the sweep of history concerns about the morality of war have gradually increased. Throughout history, war has been the source of serious moral questions. The study of warfare is known as military history.

Inventions created for warfare play an important role in advances in other fields, but modern technology has greatly increased the potential cost and destruction of war. Armies with iron weapons easily defeated armies armed with bronze. As well as organizational change, technology has played a central role in the evolution of warfare. Organization and structure has since been central to warfare, as illustrated by the success of highly disciplined troops of the Roman Empire.

The earliest city states and empires in Mesopotamia became the first to employ standing armies. War seems as old as human society, and certainly features prominently in the recorded histories of state-cultures. It must be thoroughly pondered and analyzed."---The Art of War by Sun Tzu. "Warfare is the greatest affair of state, the basis of life and death, the Tao to survival or extinction.

. A war to liberate an occupied country is sometimes characterised as a "war of liberation", while a war between internal elements of the same state may constitute a civil war. A common perception of war is a series of military campaigns between at least two opposing sides involving a dispute over sovereignty, territory, resources, religion or a host of other issues. War is contrasted with peace, which is usually defined as the absence of war.

Other terms for war, which often serve as euphemisms, include armed conflict, hostilities, and police action (note). War is a state of widespread conflict between states, organizations, or relatively large groups of people, which is characterised by the use of lethal violence between combatants or upon civilians. Space warfare. Air warfare.

Urban warfare. Mountain warfare (sometimes called alpine warfare). Sub-aquatic warfare. Naval warfare or Aquatic warfare.

Jungle warfare. Desert warfare. Ski warfare. Arctic warfare.

War where nuclear or biological weapons are not used. A war between nation-states. "crime against international peace". "police action";.

"state aggression by armed force";. "armed conflict";.

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