What are some possible reasons that women are not equally represented in every field?

[Junie]

If women do not make up 50% or so of the world's professors, doctors, scientists, (or any other occupation), does this automatically point to discrimination? Are there other possible reasons for varying percentages of female workers in each field?

I'll give one to start - chefs are overwhelmingly male - perhaps women prefer jobs which require less hours to excel, since chefs often work 70 hours a week. This could be because they want more time with family.
Is a lack of equal representation "the very definition of discrimination", as one person said in an earlier answer?