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Is heart rate correlated with birth order?

Asked by Lotti Otunnu of Lagos, Nigeria

Those who continue to believe that heart rate is correlated with birth order may not realise that such a notion is based upon only one study. The study dates back to the 1940s and involved 778 children from the town of Hagerstown, Maryland.

During routine public health screening, the heart rates of the 400 boys and 378 girls who attended one school were measured. It was found that the first-born and second-born children had statistically significantly shorter cardiac cycles, diastole, and systole compared with children who were third-born, fourth-born, or born even later in families (it's well to remember that families were often larger in those days). Strangely too, this heart rate difference was found to be greater in boys than in girls.

The heart is continually filling and emptying with blood. The "cardiac cycle" is the term for this process and consists of the complete cardiac diastole, systole, and all time intervals in-between. "Diastole" is the time at which the filling of the heart's ventricles with blood occurs. "Systole" is the contraction time at which the emptying of the heart’s ventricles occurs.

In the Hagerstown study there was no explanation for the curious finding of heart rate being associated with birth order. However, speculation included a possible previously unknown mysterious maternal factor appearing in the second, third, or subsequent pregnancy having a bearing upon the development of the child's heart.

A US Public Health Service study found no corroboration for the Hagerstown finding. A single article, authored by Antonio Ciocco, was published in Human Biology in 1943. Since then, research has failed to again find the association found in Hagerstown or anything similar.

For example, birth order was found to have no relationship to blood pressure in a 1980 study in the The American Journal of Epidemiology. That study was undertaken by Dr M Higgins and five colleagues from the US National Heart, Lung, and Blood Institute in Bethesda, Maryland, about 61 miles from Hagerstown.

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au

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