About the first participants: The first mailing to about 38,000 addresses of individual twins born in California before 1956 was sent in 1992, and 19,823 twins from 14,171 pairs responded by answering the questions. This represents a minimum of 55% of those contacted. We tried to telephone a sample of twins who did and a sample of those who didn’t answer the questions to estimate the number of letters that were never received. After doing so, we concluded that the percent who responded among those who actually received the mailing was closer to 70%.

While twin birth and death records indicate that the number of male and female twins in the population should be about equal, women are generally better about responding to requests for information than men, and we would expect that to be true of twins as well. However, because of difficulties in locating those female twins who have always used their married names, the addresses of more men than women among those born before about 1940 were identified. California-born women in this age group who are not now participating in the program are strongly encouraged to contact us. The net effect was that 56% of the same-sex pairs represented were of men, and 44% of women. One would similarly expect that roughly equal numbers of identical twins, fraternal twins of the same sex, and fraternal twins of different sexes would have responded, each amounting to approximately 16.5% of the total, and if that were true about 33% of the pairs represented would be fraternal male-female pairs. The percentages actually were those shown in the table:

Responses were not received from about 8500 twins in pairs from which the other twin did respond. In 1605 of these instances it was because the twin was dead or not known to be alive. Many of the others were not received because the other twin resided outside California or at an unknown address. Information from single twins is useful, and we encourage single twins in each of these circumstances to participate.

Geography: We start out with only a name, birthday, and address, and can estimate the rate of response only according to the address. We estimate the percent of twins responding from each part of California as follows: San Diego, Orange and Imperial Counties 49%, San Bernardino and Riverside Counties 70%, San Fernando and San Gabriel Valley regions of Los Angeles County 62%, Los Angeles County otherwise 58%, San Joaquin Valley 57%, Central Coast 62%, San Jose 52%, San Francisco and Marin Counties 78%, East Bay 62%, Sacramento Valley 62%, North Coast 64%, and Eastern Slope 70%.

Social and ethnic settings: Dividing communities by average income according to the census, the percent responding was 51%, 79%, 67%, and 74% for lowest, low, high, and highest income communities respectively. Twins from the most rural communities responded at a rate of 61%, those from the least rural at a rate of 60%. The census tells us how many resident California natives (twins and singletons) belong to each race/ethnicity group, and we can use that information to estimate the relative degree of participation. Twins who are Black, East Asian, Southeast Asian, American Indian, and Latino (including "other") participated at levels relative to the participation rate of other White twins, as follows: 60%, 53%, 79%, 74%, and 59% respectively.

Among the fraternal twin boys, 9% of had moved to different rooms by age 12, and 40% had moved to different communities (more than an hour apart) by age 25. Fraternal twin girls showed a similar tendency (8% and 40%). Whereas 6% of both the identical girls and the identical boys had moved to different rooms by age 12, and 47% of the identical girls had moved to different communities by age 25, no less than 66% of the identical boys had separated by that age.

Only 27% (male) and 38% (female) identical twins differed in height by as much as one inch, whereas 70% (male) and 73% (female) same-sex fraternal twins differed by that much. Male and female identical twins differed in weight in adulthood by as much as 10 pounds in 47% and 48% of the pairs, whereas same-sex fraternal twins differed by that much in 61% and 65% of the pairs respectively.

Education and occupation: Among all the native California twins who responded, 72% had attended at least some college, and 24% had attended at least some graduate school. When one same-sex fraternal twin attended college, the other also did 58% of the time. Among identical pairs, the other twin did so 69% of the time. When one same-sex fraternal twin went to graduate school, the other also had that opportunity 30% of the time. Among identical twins, the other did so 48% of the time.

When both members provided information about their usual occupation, 33% of identical males and 30% of identical females had similar or identical jobs, whereas 19% of same-sex fraternal males and 20% of same-sex fraternal females did so. Even among the different-sex fraternal twin pairs, 7% indicated that they held the same or a similar occupation.