Why women can't lift as much as men
It is fact that men can lift more absolute weight than women can. They are bigger than us right? However even if we compare men and women who weigh the same, the men's Olympic world record is 169KG vs the women's 123KG in the 69KG weight category.
As a general rule women are 2/3 as strong as men. However when adjusted for free fat mass, i.e. you strip everything but the muscle away, the differences disappear.
The difference in size between men and women starts to accelerate at puberty. Men get bigger and put on more muscle (thanks to testosterone), whereas women's hormones begin to become more cyclical and they store more fat. Men get their testosterone from their testicles whereas women, in the absence of testicles, get 10x less testosterone (hence why women can't 'bulk up' when you touch a free weight!). There is another reason women's hormones are thought to impair their ability to lift weights. Oestrogen can block protein damage which can be a good thing in preventing muscle soreness, but we know that there is a certain amount of muscle damage needed to encourage muscles to grow.
So if we go back to the 69KG man and 69KG women you will notice it is likely they will still look different. The man is likely to be shorter than average and more dense, whereas the women might be taller and will likely have a larger fat percentage. The man despite being the same weight, has a larger cross section of muscle, giving him the ability to lift heavier weights.
The difference in anthropometrics (body size and make up) that results from puberty, is thought to be the main reason for the difference in men and women's weight lifting abilities. When men and women's individual muscle fibres are looked at there is no difference in the quality of muscle.
Despite this finding there is a consistent observation that women are weaker in their upper body than men, whereas the lower body doesn't appear to have the same difference. This makes me feel better about my pull up struggle when I see how most men in the gym just seem to be able to jump up and pull away! It is thought to be due to the distribution of muscles in women being more bottom heavy perhaps due to our evolutionary roles not needing as much upper body strength.
There are external factors that contribute to women lifting lighter weights, which include the social desire to be strong and muscular that is more entrenched in men from a young age. The inclusion of women in weightlifting disciplines is a relatively young sport so we may see the weights women are lifting rise as the sport becomes more professional. Men's weightlifting was one of the original sports that can be traced back to the ancient Greeks and has been in the Olympics since 1896. The inclusion of women's weightlifting was at the 2000 Olympics in Sydney so we've got a bit of catching up to do!
So yes men are stronger than women in absolute terms however time may see that difference decrease as women start weight lifting from a younger age and training more professionally. Our smaller size and larger fat percentage means we'll have to put in a bit of extra grind to get that elusive pull up but being that we are mentally the stronger sex when has that ever been a problem?!
Enea, CE et al. (2011) Circulating adnrogens in women: Exercise induced changes. Sports medicine 41, 1-15
Miller, A.E.J. et al. "Gender differences in strength and muscle fiber characteristics." European Journal of Applied Physiology and Occupational Physiology. Vol. 66, Issue 03. March 1993. (Nov. 28, 2012)
Lauback, L (1976) Comparative muscular strength of men and women: A review of the literature. Aviat. Space Environ. Med 47, 354-542
Parker-Pope, Tara. "Why Women Can't Do Pull-Ups." The New York Times. Oct. 25, 2012. (Nov. 28, 2012) http://well.blogs.nytimes.com/2012/10/25/why-women-cant-do-pull-ups/?ref=health