Until 1969, college students were excempted from being drafted into the Vietnam War. This was considered unfair because if favored high and medium income men who could afford college. Those who can't afford college belong disproportionally to low income populations.
After 1969, the daft was changed. The lottery was a radomnized processed but it still had flaws (it disproportionally drafted men born in the latter months of the year).
