Scientists have proposed sex chromosomes to be the real reason behind why women live longer lives on average than men.
Chromosomes are structures that bear the DNA in the nucleus of cells. Humans have 23 pairs of chromosomes in their cells. One of these pairs is the sex chromosomes. These are the X and Y chromosomes. Females have 2 X chromosomes while males have one X chromosome and one Y chromosome.
Unguarded X Hypothesis
According to this hypothesis, in males, the Y chromosome is less capable of protecting an individual from the effect of harmful genes that may be expressed on the X chromosome. The Y chromosome’s smaller size is a contributing factor. In case of females, the harmful effects arising from genes on one X chromosome can be countered by the other healthy chromosome.
Other organisms also have sex chromosomes. Depending on whether they have 2 same type of chromosomes (eg: XX in females) in the pair or different (eg: XY in males), they are considered a homogametic or heterogametic sex respectively. However, the sexes affected are reverse in some cases. In all cases, heterogametic sex dies earlier on an average than the homogametic sex.