Biological twins is believed to be a result of a very early split, “probably during blastomere, gastrula, or early embryonic axis stage, less than two weeks from conception” (Koch, 1966, pg 6). This happens in a cell mass that would have developed into just one individual. But if the cell mass is duplicated, this results in an identical , monozygotic (MZ), or one-egg type twins that has been occasioned by early fission and have identical genes. All terms used interchangeably. Also, it has been speculated that twinning occurs because of the mother and it occurs from her matrilineal line (Koch, 1966).

All biological twins differ in physical characteristics just a small percentage. This includes height, muscularity, size of hand, and size of head (Conroy, 2000). Studies have been done concluding that 64% of the time the first born is much heavier of the two. The first-born that is larger of the two tend to be in better condition. Also, it has been exhibited that same identical twins who are male have more chances of having defects during pregnancy than females. This is due to extreme prematurely at birth, but it is still unknown of how this occurs in the womb. (Koch, 1966). As twins grow they begin to experience their individuality. Although, twins have similar characteristics, it is a fact that their weight and intelligence differ greatly from one another. The scores from Binet IQ and Stanford Achievement Test showed that twins do not have the same intelligence and this is due to the environment. This also concludes that personality and education achievement is greatly influenced by the environment. Twins may be similar on the outside, but they are different in the inside (Conroy, 2000).

Monozygotic twins are very alike because of their genes, but this could be a problem (Conroy, 2000). The modal concordance done for identical twins resulted in males having 95% concordant and for the females 93% (Koch, 1966). “MZ twins show a relative rather that a perfect degree of concordance for [some] pathological traits,” (Gedda, 1961, pg 99). An example of this is when one twin may have the symptoms of a disease but the other twin may have undeveloped form. This is very common especially in twin pathology. But in some cases, diseases such as measles, diabetes mellitus, and rickets are the three most common incident of concordance. Also, studies have shown that 89% of biological twins will share Down’s Syndrome disorder to their twin brother or sister. With respect to Psychological disorders, there are four most common disorder evident of resemblance from both twins. Feeblemindedness shows 94%, schizophrenia 80%, manic-depressive psychosis 77%, and criminality 68% are concordant between identical twins (Conroy, 2000). According to other studies, habits such as smoking shows 99%, coffee drinking 94% , and alcohol drinking100% which are higher levels of concordance (Conroy, 2000).