1. I think the original is clearer. The comparison of one third with under two thirds seems a little odd, since one third is under two thirds. Do you know the actual numbers that this refers to? If it is very close to two thirds, you might say, "just under two thirds". 2. I think "of both genders is better". The term "manual socio-economic groups" seems somewhat add. I assume you mean the socio-economic group including unskilled, manual workers. In that case, you might say, "it became increasingly associated with the socio-economic groups including unskilled workers of both genders."

1. Grammatically speaking, you *could* but shouldn't change the sentence to: "30 years ago, about a third of professional men smoked (which was) compared with under two-thirds of unskilled manual workers." Reason: If you use the passive voice ("...which was compared with..."), it strongly emphasizes the ACTION of comparison: somewhere, a group of researchers looked at the data and compared the two. While this is grammatically and logically correct, it's not the right emphasis for the sentence. The writer wants to make the comparison between the two groups here and now in the present. When the comparison was made and by whom is irrelevant to the writer's purpose in this sentence; avoid the passive voice and leave the sentence as is.

2. You *could* change the sentence to "...in both genders." The preposition "in" can be used for groups, and genders can be thought of as groups. This would be a formal, more academic way of speaking though: "in a group" is more commonly used to refer to the particular members of a group ("I am in this group, you are in that group."). "Socioeconomic groups" aren't really members of genders - men and women are. Scientists will often use "in" when referring to groups in experiments (Example: "Increasing height was associated with increasing strength in both the control and experimental groups."), so if you write "in both genders," you are treating the two genders like different groups in a scientific experiment. Not wrong, but a little formal.