In major athletics events (Olympic Games, World Championship), men run a decathlon (10 events) and women run a heptathlon (7 events).
Why isn't the combined discipline the same for both genders? I understand that individual events are adapted (like 110m hurdles for men and 100m hurdles for women, or a 800g javelin for men but 600g for women), but why don't men run an heptathlon or women a decathlon instead?
The actual answer to this is "History". The women's heptathlon was only introduced in the Olympic program in 1984; before that women contested a pentathlon (five events) when they were allowed to compete in multi-events at all.
Bear in mind that almost all advances toward event equality between men and women have happened in the last half-century, and that the glacial pace of change in international bureaucracies still dominated by older men means there is still work to be done. 1984 was also the year of the first women's marathon, and long-distance track races (e.g. 5,000m/10,000m) didn't achieve parity until 1996; the women's 3000m steeplechase didn't make its Olympic debut until 2008.
Taken as an aggregate, there are physiological differences between men and women (e.g. leg length - the average man has longer legs than the average woman) which lead to different event specifications, such as the height of hurdles and the distance between them.
In an event with as many variables as the multis have (event distances, hurdle heights, implement weights, etc.) the negotiation of which of these variables are affected by measurable physiological differences and which are simply holdovers of history gets really, really complicated.
Nice question, I haven't thought about it before. There are scoring tables and also women who do the decathlon (Source) and I'd say that a woman competing in heptathlon could generally also do a decathlon as far as fitness is concerned. The same is of course true for a male decathlete doing a heptathlon (as they do indoors).
I guess the main reason why the current system is not changed is that athletes training for a specific number of events can't train for three more events and still be maintain a similar level in all of them. So a sudden change would mean a phase to adapt for the top athletes and thus decrease the appeal of the sport.
