Then it's Tommy this, an' Tommy that, an' "Tommy, 'ow's yer soul?"
But it's "Thin red line of 'eroes" when the drums begin to roll,
the drums begin to roll, my boys, the drums begin to roll,
O it's "Thin red line of 'eroes" when the drums begin to roll.
– Kipling
The poverty stricken agricultural societies scattered across human space made fertile recruiting grounds for the mercenary regiments. They tended to leave with as many troopers as which they arrived from every planet they fought upon regardless of losses. Regiments generally preferred farm boys as recruits to the urban slum dwellers of Old Earth or the more urbanised colonies because they had fewer psychological problems; oddly enough sociopaths, psychopaths and gangsters tend to make bad soldiers.
Farming families in the agricultural colonies tended to be large despite the high infant mortality rate; children were needed to work the farm. However, eventually only one offspring and spouse could inherit or the farm would have to be split into uneconomically small units. The options for younger sons were limited. A trooper's monthly pay in one of the elite regiments was higher than a year's salary for an agricultural labourer. Troopers also got access to medical care and pensions if they survived. The death rate among the dirt-poor farmers did not compare particularly favourably with soldiers.
Finally, soldiering had a certain romantic wickedness to it. The image of a swooning exotic girl on every planet just ready to be bowled over by a likely lad in a uniform stuffed with money to spend held a considerable attraction, at least for the male recruits.
If a recruit was lucky, they ended in an elite mechanised regiment where they fought protected by the thickest iridium armour, the best electronics and the most powerful guns the galaxy had ever seen. They also received an education, maybe for the first time in their lives. If they were unlucky, they spent their days as paramilitary policemen in ill fitting uniforms, that sometimes still bore bloodstains from the previous owner, gunning down rioters in shantytowns.
Officers in the regiments came from a variety of backgrounds. If the unit was mono-ethnic, the officers might be drawn from the traditional upper classes. Sometimes they were businessmen, protecting their investment. In the best regiments where results counted more than fashionable accents, officers were promoted from below.
As long as business was good, and there was never enough productive farmland to go round, the recruits came whatever their reception.
Extract from Psychology Of A Hired Gun, Fin Sao