Sir,

A recent paper in European Psychiatry by Caci et al. Reference Caci, Robert and Boyer[1], showed that the degree of morningness was negatively correlated with the personality traits of novelty seeking and persistence from Cloninger’s TCI Reference Cloninger, Svrakic and Przybeck[4]. In their discussion, however, they unfortunately misinterprete our earlier paper Reference Chotai, Forsgren, Nilsson and Adolfsson[3], and thereby fail to recognize that our various published results in fact point towards the same conclusions as theirs.

In Chotai et al. Reference Chotai, Forsgren, Nilsson and Adolfsson[3], it was shown that adults born during the winter months October to January were less likely to be high in novelty seeking or in persistence than those born during the rest of the year. This was indicated by the odds ratio of 0.75 for women regarding novelty seeking, and the odds ratio of 0.64 for men regarding persistence, for “NS high” in the middle set of columns of table 2 in Chotai et al. Reference Chotai, Forsgren, Nilsson and Adolfsson[3]. The study by Natale and Adan Reference Natale and Adan[5] had shown that persons born during autumn and winter were high in morningness, compared to those born in spring and summer. So these two results suggest that the degree of morningness would be expected to be negatively correlated with novelty seeking and persistence, as found by Caci et al. Reference Caci, Robert and Boyer[1].

In fact, we have also published further analyses that would support an inverse relationship between the degree of morningness and the degree of novelty seeking. We have performed nonlinear regression analyses on the curves of the relevant variables (e.g. degree of morningness or novelty seeking) according to the month of birth, to fit a cosine curve with one cosine cycle per year of birth. This yields regression estimates of the birth month giving the highest peak and the birth month giving the lowest peak of the cosine curve for the relevant variable. For morningness, we obtained the lowest degree for the summer-borns around June and the highest for the winter-borns around December Reference Natale, Adan and Chotai[6]. For the data of Chotai et al. Reference Chotai, Forsgren, Nilsson and Adolfsson[3], we found that novelty seeking obtained maximum for the summer-borns around May and minimum for the winter-borns around November Reference Chotai and Adolfsson[2].

Caci et al. Reference Caci, Robert and Boyer[1] studied only male subjects. In view of the stronger relationships found among women compared to men in our studies regarding the relationship of the season of birth with novelty seeking and morningness [2,6], the results of Caci et al. Reference Caci, Robert and Boyer[1] are likely to be valid even for women.