If you're looking at country-level data it may be too broad and so harder to identify, only one order away from pointlessly looking for the same effect in "the whole world". You need granularity, where the effects are seen.

For example, small UK villages presently having emotion-filled community meetings unable to cope with the negative effect of sudden influxes of 1000's of allotted refugees radically changing every aspect of their home.

But why require evidence, when the basic math has it:

You have x number of people with y number of resources. What will happen if you increase x without also increasing y?