When the draft is in force, the army gets a broad spectrum of the population, not just those who want to be soldiers or have few other options. Therefore a period in the not too distant past when there was a draft provides a good comparison base for the present generation of recruits.
The Vietnam war is going back a bit far but the last person drafted in the USA was in 1973 and it's been voluntary ever since. For the UK, national service ended in 1960 with the last servicemen leaving in 1963 so we are going back a bit.
In the years after Vietnam there have been periods when average IQ scores of volunteer army recruits have been lower than those who were drafted which isn't a good indication, particularly as the draft favoured poor and uneducated while the more intelligent got deferements and went on to further education. There were also times when a judge would give the option to someone who had just been found guilty, of joining the army or going to prison. Then again there were times when the military wouldn't take you if you had an outstanding parking fine.
This article regarding the quality of current US recruits is quite interesting and dispels a few myths.
https://www.psychologytoday.com/sg/b...lowest-our-low
It will be interesting to see the quality of recruits in the years following this pandemic, with fewer opportunities available the prospect of secure employment, paid training and opportunities for advancement that the military provide would be very attractive to someone who wouldn't have otherwise considered the army.