Both India and Pakistan are proven nuclear powers after last year's tit-for-tat tests, and it is believed they may each possess a small number of nuclear warheads.

BBC: Nuclear clash came 'very close'

DSU's recommended penalties are structured, tit-for-tat is a likely outcome.

ECONOMIST: Trade

The trial heard that Mr Greene's killing was a result of a spiralling tit-for-tat war between two groups of young men from a small area of north-west London.

BBC: RELATED BBC SITES

But Pakistan's refusal to play appears to be a tit-for-tat response after India said they would not take part in a Test and one-day series in Pakistan in May.

BBC: Pakistan withdraw from Asia Cup

The Russian deputy foreign minister on Tuesday vowed a "targeted and appropriate" response to Britain's decision to expel four Russian diplomats in a tit-for-tat battle over the murder investigation of former KGB officer Alexander Litvinenko.

NPR: Kremlin Issues Warning After Expulsions

One social media expert told the BBC that the tit-for-tat action had been motivated by Instagram's desire to make money.

BBC: Twitter icon, smartphone