Nancy Krieger, professor of society, human development and health at the Harvard School of Public Health and colleagues, compared the health status, education levels and adult occupational class among women who were identical twins and who lived together through adolescence, to demonstrate that adult socioeconomic conditions affect adult health, above and beyond genetics and early life conditions.
They found that identical twins with different adult occupational class positions (working class versus professional) differed considerably in a range of health outcomes, including blood pressure, cholesterol levels, and self-rated health, whereas twins with the same adult occupational class had similar health, as did twins who had the same or different educational level.
Because educational level is attained at an earlier age than adult occupational class, the study provides novel evidence that lifetime socioeconomic position – including after educational attainment – influences adult health. The results appear in the July 26, 2005, issue of the Public Library of Science Medicine, (www.plosmedicine.org).
Studying the health status of twins as adults, particularly identical twins, can provide unique insights to the issue of the influence of adult experiences, above and beyond early environment and genetics, to health outcomes later in life. Identical twins share not only similar genetics but the same environment while in childhood. Comparing the health of identical twins who have the same versus different educational level and adult occupational class can help answer the question of how much adult socioeconomic conditions affect adult health.
Participants for this study were selected from the Kaiser Permanente Women Twins Study Examination II. The researchers gathered information on 308 twin pairs (178 identical twin pairs and 130 fraternal twin pairs) who lived together until at least the age of 14, via questionnaires to assess childhood and adult occupational status and physical examinations to assess health measures such as blood pressure, cholesterol levels, body mass index (BMI) and physical activity. The twins' adult occupational position was categorized as either working class (non-supervisory employee) or professional/non-working class (owned a business, self-employed, employed or supervised others).
The researchers found that identical twins who had different adult occupational position had very different health outcomes, compared to twins in the study who had similar adult occupational position. Working class twins compared to their professional/non-working class sibling had significantly higher blood pressure, higher diastolic blood pressure, higher BMI and higher levels of low-density lipoprotein cholesterol. Additionally, identical twin pairs who had different adult occupational class were nearly 4 times more likely to rate their health differently, compared to twins with the same adult occupational class.
"The debate about what influences an adult's health status has been going on for quite some time," said Krieger. "Is it only childhood living conditions or education level or does occupational position later in life also matter? In this study we found that among identical women twins, their adult occupational position as an adult was an important predictor of health status as an adult, regardless of the environment they grew up in." She continued, "These findings have important policy implications with regard to directing resources to improving health for both children and adults, since they underscore the need for improving socioeconomic conditions across the lifecourse."