Section A What determines the kind of person each of us develops into over a lifetime? How much are we influenced by our home environment, and how much are we influenced by our genes? (86) One way that psychologists have tried to answer these questions is by studying twins : identical twins ( coming from one fertilized egg, so sharing the same genes) and fraternal twins ( coming from different eggs, so sharing 50% of their genes). By comparing these two groups, researchers hope to be able to estimate the influence of genes versus the influence of environment on the development of many human traits, such as personality, talents, interests, and attitudes. Psychologists from the University of Minnesota have conducted several different studies with over 8,000 pairs of twins and their families. (87) To determine what accounts for differences between twins, they have compared identical twins who were raised together with identical twins who were separated at birth and raised in different families. Their assumption is that any similarity in twins who are raised together can be attributed to both genes and environment, whereas similarities in twins raised apart are clue only to genes, since they had different environments while growing up. Their studies indicate that genes have about a 50% influence on our personalities, while our environment accounts for the other 50%. (88) Researchers in this study have also tried to test the theory that we each develop a set of criteria for choosing a mate based on our genes and our unique experiences growing up. To do this, researchers compared the spouses of male identical twins who were raised together. They found that the spouses of these identical twins were not much more similar to each other than random pairs of people. They also found that two-thirds of the twins were 'indifferent' to their twin's mate or 'actively disliked' her. These results indicate that neither our genes nor our upbringing is a good predictor of the spouse we choose. Instead, researchers believe this choice appears to be random, and based more on whom we are near when we are ready to fall in love. Another question this study has sought to answer is whether twins become more or less similar as they mature. They found that as identical twins age, they become less similar in their physical appearance, but more similar in their abilities, such as mathematical skills. On the other hand, as fraternal twins age they become less similar in these abilities. Some psychologists are skeptical of twin studies. (89) These critics question some of the assumptions made by twin researchers, such as the assumption that identical twins and fraternal twins have similar environments, or that identical twins raised apart are raised in very different environments. Some believe that, even in the same environment, there can be disparities in the way this environment affects siblings. As a result, it is difficult to determine clearly how much of our personality is genetically inherited and how much is determined by our environment. (57)