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The vital statistics show that human females outlive males at every biological stage. Once the embryo arrives in the uterus, more males die at every stage, at least until the eighth decade when the majority of survivors are female. Unexpectedly, the same statistics also show that more boys are born than girls, which is difficult to explain, because the sperm that determine the sex of the fetus, are not skewed toward males. Recently, new data reveal the reason for the increased number of male births; they imply the significant loss of females before the fetus arrives in the womb. Thereafter, there is an excessive loss of males – not only in utero but throughout their lives. One likely reason for the sex differences in fetal survival is the way that humans compensate for the sex difference in number of X chromosomes.