Boundaries of I-function in twins Identical, conjoined, and half-twins are all examples of intrinsic variability in humans. Intrinsic variability exists in all animals and is an adaptive mechanism built into the nervous system in response to inputs. This mechanism allows humans to distinguish the same inputs as different from each other and therefore the possible outputs vary over time. It is possible that, due to identical genetic input, twins may share identical neural pathways and identical I functions. This hypothesis could explain the identical behaviors and interconnectedness of feelings and thoughts shared by twins. The observed differences in twins' behaviors could simply be due to inherent variability causing different outcomes or behaviors. During ovulation, when a woman releases an egg, three different processes can occur. First, the egg can remain intact until the uterus where, if not fertilized, it will be eliminated from the body together with the unused endometrium. Second, the egg, if fertilized, can develop into a single embryo, which is the most common type of pregnancy in humans with approximately 99% of all singleton births (1). Eventually, the egg can divide into two separate halves resulting in genetically identical twins. The three types of twins mentioned above are identical, which are created when the fertilized egg separates into two complete, identical, conjoined parts, which occurs due to the fact that one fertilized egg does not complete the equal separation resulting in two fetuses fused together in some way and half-twins or polar-body twins, which form when an unfertilized egg splits into two complete, identical parts and is then fertilized by two different sperm. The frequency of identical twins is 3.5 in 100 births (1) and the frequency of conjoined twins is 1 in 50,000 births with only 100 known cases surviving the first year (5). There is no scientific explanation for why an egg divides into two or more parts. It has been observed that egg splitting can be hereditary but can also occur spontaneously. This unexplained spontaneity or divergence from normal egg growth and fertilization could be due to intrinsic variability. This intrinsic variability could also be linked to some differences in behavior of two genetically and culturally identical twins. Ideally, one might say that twins with identical genetic information and environments would have extremely similar, if not the same, neural pathways and central pattern generators.).