Identical twins share 100% of their DNA. Full siblings are first-degree relatives and, on average, share 50% of their genes out of those that vary among humans, assuming that the parents share none of those genes. Half-siblings are second-degree relatives and have, on average, a 25% overlap in their human genetic variation.
Full siblings (full brothers or full sisters; or brother and sister) have the same biological parents and are 50% related (full siblings share 50% of their genes out of those that vary among humans). Identical twins by definition are 100% related.
There are two types of twins: identical and fraternal. Identical twins have exactly the same genes; fraternal twins are no more similar than regular siblings. Often, twins with a close relationship will develop a twin language from infanthood, a language only shared and understood between the two. Studies suggest that identical twins appear to display more twin talk than fraternal twins. At about 3 years of age, twin talk usually ends.
Researchers were interested in subjects who were in the later years of life. They knew that past studies suggested that genetics played a larger role in one's personality in the earlier years of their life. However, they were curious about whether or not this was true later on in life. They gathered subjects with a mean age of 59, who included 99 pairs of identical twins, and 229 pairs of fraternal twins who were all reared apart. They also gathered twins who were reared together: 160 pairs of identical twins, and 212 pairs of fraternal twins. They studied the most heritable traits in regard to personality, which are emotionality, activity level and sociability; also known as EAS. This study found that identical twins resembled each other twice as much as fraternal twins, due to genetic factors. Furthermore, environment influences personality substantially; however, it has little to do with whether they are reared together or apart. This study also suggests that heritability is substantial, but not as substantial as for younger subjects; it has less significance later on in life.
Half-siblings are people who share one parent but not both. They may share the same mother but different fathers (in which case they are known as uterine siblings or maternal half-brothers/half-sisters), or they may have the same father but different mothers (in which case, they are known as agnate siblings or paternal half-brothers/half-sisters. In law, the term consanguine is used in place of agnate). They share only one parent instead of two as full siblings do and are on average 25% related.