The tortured question was put to rest in 2003 by Britain’s Royal Society of Chemistry. MIF (milk in first) creates a cup of tea that’s smoother and richer; MIL, a cup that’s more tannic.
The chemical explanation involves the degradation of milk proteins. “If milk is poured into hot tea, individual drops separate from the bulk of the milk and come into contact with the high temperatures of the tea for enough time for significant denaturation to occur. This is much less likely to happen if hot water is added to the milk,” it reported.
There are additional cultural and historical factors fuelling the MIF/MIL debate, but ultimately it comes down to a question of taste.