Table 1.
Description of Australian, Dutch and US Samples
| Australian | Dutch | US | |
|---|---|---|---|
| N individuals by twin type | |||
| Females from SS pairs | 323 | 137 | 1519 |
| Females from OS pairs | 153 | 57 | 703 |
| Total | 476 | 194 | 2222 |
| Year of birth (range) | 1897–1939 | 1917–1954 | 1894–1937 |
| Religion (%) | |||
| Catholic | 16.8 | 36.6 | 16.5 |
| Protestant | 70.1 | 40.7 | 71.4 |
| Jewish | 0 | 0 | 4.3 |
| Other | 1.7 | 2.6 | 3.1 |
| None | 5.9 | 8.8 | 2.6 |
| Not answered | 5.5 | 11.3 | 2.2 |
| Education (%) | |||
| High school diploma or less | 63.2 | 60.5 | 16.6 |
| Education post high school | 36.8 | 39.5 | 83.4* |
| Reproductive variables | |||
| Mean number of years married (SD) | 29.1 (12.5) | 32.4 (10.1) | 34.9 (10.5) |
| Ever used birth control (%) | 35.9 | 70.2 | NA |
| Stepchildren in the household (%) | 2.8 | 1.6 | NA |
Note: NA = not available;
*
A subset of participants in the Virginia 30,000 study were recruited through an advertisement in an American Association of Retired Persons newsletter. The majority of data used in this study comes from this subset which is known to have a higher education level than the general American population (Davies & Love, 2002; Truett et al., 1994).