Why do rival parties' manifestos so often end up sounding alike?
If voters are spread along a line from left to right, a party that edges towards the centre keeps its own wing and gains centrists. Both parties face that pull, so both end up near the middle.
Parties chasing votes are pulled towards the voter in the middle
At A-level, government failure is a list of ways an intervention can misallocate resources. Public choice asks why: it applies the economist's assumption of self-interest to voters and politicians. Line voters up from left to right on a single issue, such as how much to spend on public services, and suppose each votes for the party nearer their own view. A party that moves a step towards the centre keeps its own wing and wins voters from the middle. With two parties, both are pulled towards the median voter, the one with as many voters to the left as to the right.
Far apartParty A stands on the left and party B on the right. Each wins the voters nearer to it, and the voters between them split at the halfway point.