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The human twin birth rate in the United States rose 76% from 1980 through 2009, from 9.4 to 16.7 twin sets (18.8 to 33.3 twins) per 1,000 births. [5] The Yoruba people have the highest rate of twinning in the world, at 45–50 twin sets (90–100 twins) per 1,000 live births, [6] [7] [8] possibly because of high consumption of a specific type of yam containing a natural phytoestrogen which may ...
Identical twin studies are a useful mechanism for assessing the role of genes and environment. Twin studies have also found that among twins with differing sexual orientations, homosexual twins were significantly more gender nonconforming than their heterosexual co-twin, and that this was noticeable from a young age. [29] Bailey states:
Sexual selection is quite different in non-human animals than humans as they feel more of the evolutionary pressures to reproduce and can easily reject a mate. [2] The role of sexual selection in human evolution has not been firmly established although neoteny has been cited as being caused by human sexual selection. [ 3 ]
Heteropaternal superfecundation is common in animals such as cats and dogs. Stray dogs can produce litters in which every puppy has a different sire. Though rare in humans, cases have been documented. In one study on humans, the frequency was 2.4% among dizygotic twins whose parents had been involved in paternity suits. [6]
A term most applicable to vertebrate species, multiple births occur in most kinds of mammals, with varying frequencies. Such births are often named according to the number of offspring, as in twins and triplets. In non-humans, the whole group may also be referred to as a litter, and multiple births may be more common than single births ...
The common pathway of sexual differentiation, where a productive human female has an XX chromosome pair, and a productive male has an XY pair, is relevant to the development of intersex conditions. During fertilization, the sperm adds either an X (female) or a Y (male) chromosome to the X in the ovum. This determines the genetic sex of the embryo.
Intersex people have been treated in different ways by different cultures. Whether or not they were socially tolerated or accepted by any particular culture, the existence of intersex people was known to many ancient and pre-modern cultures and legal systems, and numerous historical accounts exist.
The term gonochorism is most often used for animal species, an estimated 95% of which are gonochoric. [17] It is very common in vertebrate species, 99% of which are gonochoric. [18] [19] Ninety-eight percent of fishes are gonochoric. [20] Mammals (including humans [21] [22]) and birds are solely gonochoric. [23] and Tardigrades are almost ...