Why is it that for a woman to find her size in a man’s shoe she must subtract 2 and vice versa? Why not have a universal size for both?
[And I thought it had something to do with television.]
Yeah of all the places for me to make a typo I make it there. 
For the same reason a woman must buy the rest of her clothes by a complex code that is basically a rough guess, but a man can buy his by clearly defined body measurements. And why she will pay more to have a shirt washed and pressed than he will. And why, except at Nordstrom’s, she must pay extra to have the garment length adjusted, but it’s usually free for a man. It’s a vast conspiracy. I haven’t decided if it’s a left-wing or a right-wing one, but it’s a vast conspiracy nonetheless.
Women will actually take the smaller numbered size when presented with two shoes, even if the shoe hurts to wear. Why that is so is a mystery, but the fact that it is so is quite well known to shoe sales professionals. The trick of lying to the customer about the size of her feet became so widespread, that it was regularized by the manufactures.
The same exact length in a woman shoe will have one full size difference than that of a man. The same proportionate width will have a full letter sized designation less.
Women pay more for these shoes too; so as to be sure that they are better quality. Since they generally never wear them more than a dozen times in their lifetime, actual quality matters very little difference.
Comfort, of course is not a factor at all.
The above applies primarily to “high fashion” shoes, although the sizes are transferred to more mundane ladies footwear.
Tris.
I believe most countries use the same sizes for men and women. Some time ago I did some research on this topic for a travel agent who organized tours in Spain and got a number of complaints from Americans who said they could not get shoes in Spain which would fit them. This is from the report I wrote:
Assuming American shoe sizes increment in 1/2
of nominal size (8, 8.5, 9, etc) and Spanish
increase in integral sizes (39, 40, 41, etc),
then the size ratio is about 2/3, i.e.: an
increase in 2 Spanish sizes (35 - 37) is
equivalent to 3 American increments (5 1/2 - 7).
This means American increments are smaller (2/3)
and one can get a better fit with American sizes.
Shoe sizes USA USA
Cm UK EU Spain Men Women
24.0 3.0 35.5 34.4 3.0 5.0
24.5 3.5 36.1 35.0 3.5 5.5
25.0 4.0 36.7 35.6 4.0 6.0
25.5 4.5 37.3 36.3 4.5 6.5
26.0 5.0 37.9 36.9 5.0 7.0
26.5 5.5 38.6 37.6 5.5 7.5
27.0 6.0 39.2 38.2 6.0 8.0
27.5 6.5 39.8 38.9 6.5 8.5
28.0 7.0 40.4 39.5 7.0 9.0
28.5 7.5 41.0 40.2 7.5 9.5
29.0 8.0 41.6 40.8 8.0 10.0
29.5 8.5 42.2 41.5 8.5 10.5
30.0 9.0 42.8 42.1 9.0 11.0
30.5 9.5 43.4 42.8 9.5 11.5
31.0 10.0 44.1 43.4 10.0 12.0
31.5 10.5 44.7 44.1 10.5 12.5
32.0 11.0 45.3 44.7 11.0 13.0
32.5 11.5 45.9 45.4 11.5 13.5
33.0 12.0 46.5 46.0 12.0 14.0
To convert Spanish size to US (men) size:
Subtract 30.5 then multiply by 7/9 (approx 3/4)
(For US women now add 2)
To convert US (men) size to Spanish
(For women's size first subtract 2)
Multiply by 9/7 (approx 4/3), then add 30.5