Month of birth in relation to suicide

Salib & Cortina-Borja ¹ find that persons born during the spring–summer season of April, May and June were significantly more likely to die by suicide than those born during other months: they find a peak for May and a trough for October.

However, they misreport our earlier results in this field when they state in the introduction that ‘Chotai et al ² reported that people born in winter in Sweden were significantly more likely than those with other birth seasons to have used hanging as a suicide method’. They further misreport earlier findings of ours when they state in the discussion that: ‘… winter variations in serotonin reported by Chotai & Åsberg ³ are inconsistent with the findings of this study, essentially the opposite of the Swedish findings’.

Our earlier findings are in fact similar to and consistent with the results of Salib & Cortina-Borja. In Chotai et al ² we clearly show that those who preferred hanging rather than poisoning or petrol gases were significantly more likely to be born during February–April. In Chotai &Åsberg ³ we demonstrate that those born during February–April had significantly lower levels of 5-hydroindoleacetic acid (5-HIAA).

We have also published cosine analyses of our data, ⁴ in which we found that the minimum of the month-of-birth curve for 5-HIAA was obtained for the birth month April (t-min 3.4, Table 1, where the interval 3–4 depicts April) and the maximum was obtained for October (t-max 9.4). We also reported that the maximum of the month-of-birth curve for preferring hanging was for March–April and the minimum was for September–October.

Low serotonin turnover has been implicated as a risk factor for suicidal behaviour, particularly with violent or lethal methods of suicide, as discussed by Salib & Cortina-Borja. ¹ Thus, our findings are in line with those of Salib & Cortina-Borja regarding suicidality, since we obtained a peak for the birth month April comparable to their peak for May, and found a trough for 5-HIAA for the birth month April.

In another epidemiological study, ⁵ we report that season of birth association with suicide methods is found in those without a history of psychiatric contacts, but not in those with such a history. We have argued that season of birth associations for suicide methods are likely to be mediated to a large extent by a suicidality trait independently of specific major psychiatric disorders, with serotonin as the likely underlying neurotransmitter.

In our studies, the season of birth variation was found for hanging as the suicide method, but not for other methods often denoted as violent, for example firearms or drowning. Hanging is a more universal method of suicide, and gender differences in the proportion of hanging are much lower than for other methods. In this light, it would be of interest to analyse the data of Salib & Cortina-Borja, specifically with regard to whether there is a month of birth variation in suicide by hanging.