To what extend may european colonisation and slave trading be argued as responsible for a current lack of development in what was formerly called the third world?

On way in which colonization and slave trade can be blamed for the current situation is that the colonial powers have divided the land arbitrarily, and this means  that today many African countries are facing many problems of ethnic conflicts.

Also, slavery and colonization meant that for many years the colonies were drained of resources, but not invested into: so they don't have as good infrastructure  as they maybe would have without having been colonies.