Association between sexes of successive 1 siblings in data from Demographic and 2 Health Survey program

6 Prediction of future child’s sex is a question of keen public interest. Probability of having a child of 7 either sex is close to 50%, although multiple factors may slightly change this value. Some 8 demographic studies suggested that sex determination can be influenced by previous pregnancies, 9 although this hypothesis was not commonly accepted. This paper explores correlations between 10 siblings’ sexes using data from Demographic and Health Survey program. In the sample of about 11 2,214,601 women (7,985,855 children), frequencies of offsprings with multiple siblings of same sex 12 were significantly higher than can be expected by chance. These deviations of frequencies were 13 driven by positive correlation between sexes of successive siblings (coefficient = 0.065, p < 0.001), i.e. 14 a child was more likely to be of same sex as preceding sibling. This correlation could be caused by 15 secondary sex ratio adjustment in utero since the effect was decreasing with length of birth-to-birth 16 interval, and birth-to-birth interval was longer for siblings with unlike sex. A variation of sex ratio 17 between families was present as well. 18