After 1857 the Indian Army was rebuilt on the doctrine that certain communities were inherently fit for soldiering and the rest were not. Punjabis, Sikhs, Dogras, Pathans, Gurkhas and a short list of others were classed as martial races. Bengalis, Tamils and most of the rest of India were not.
The doctrine had no basis beyond the political convenience of recruiting from the areas that had stayed loyal in 1857. It was formalised by Lord Roberts and set out in recruiting handbooks that described the physical and moral characteristics of each class.
The effect on Punjab was total. Whole districts became recruiting grounds. Rawalpindi, Jhelum, Ludhiana, Amritsar and Hoshiarpur sent men by the tens of thousands. Land grants in the canal colonies were tied to military service. Pensions became a rural income stream and remained so.
In the First World War Punjab supplied around 350,000 combatants, roughly half of India's total, from a province with about a twentieth of its population.
The theory was abandoned as doctrine after 1947 and its recruitment patterns were not. Punjab's overrepresentation in the Indian and Pakistani armies is a direct inheritance.
The theory held that certain communities were naturally suited to soldiering and others were not, and after 1857 recruitment was reorganised around it.
Punjab was the main beneficiary. By 1914 the province supplied a very large share of the Indian Army from a small share of the population, concentrated in particular districts and communities.
It was a recruiting policy dressed as an anthropology. Its consequences, in landholding, in remittance, in emigration and in politics, run through the whole modern history of the province.