The research by scientists at the University of New South Wales (UNSW),Sydney,  published recently in the journal Biology Letters, has tested one of many hypothesis and have proven that the real reason why women outlive men is related to the sex chromosomes.

Findings of the hypothesis

  • Across that broad range of species, the heterogametic sex (XY in humans) does tend to die earlier than the homogametic sex (XX in humans), and it’s 17.6 per cent earlier on average.
  • While the pattern is the same across species, the sexes affected are sometimes the reverse. 
    • In birds, butterflies and moths: In these cases it is the male that has an identical pair of sex chromosomes (ZZ) while the female has ZW chromosomes. 
    • Female birds, butterflies and moths were usually found to die earlier than their male counterparts, giving more credence to the unguarded X hypothesis (strictly speaking, unguarded Z in this case).
  • In species where males are heterogametic (XY), females live almost 21% longer than males. But in the species of birds, butterflies and moths, where females are heterogametic (ZW), males only outlive females by 7%.


Suggestion given by the hypothesis:Unguarded X hypothesis

W.r.t Males

W.r.t Females

  • The Y chromosome in XY is less able to protect an individual from harmful genes expressed on the X chromosome. 
  • As the Y chromosome is smaller than the X chromosome,in a male, it is unable to “hide” an X chromosome that carries harmful mutations.
  • This may later expose the individual to health threats.
  • There is no such problem in a pair of X chromosomes (XX) in a female. 
  • Even if one of the X chromosomes has genes that have suffered mutations, then the other X chromosome, which is healthy, can stand in for the first, so that the harmful genes are not expressed. 
  • This maximises the length of life.




Chromosomes are thread-like structures that hold the genes.

It is the genes that determine the various traits of an individual , including eye colour, blood type and sex.

Each chromosome is passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique.



Inside the nucleus of animal and plant cells.





It is  made of protein and a single molecule of deoxyribonucleic acid (DNA).

Other details

The human cell has 23 pairs of chromosomes. 

One pair is of the sex chromosomes, named X and Y, which determine whether an individual is male or female. 

A female has two X chromosomes (XX) while a male has one X and one Y (XY).