If somebody roams into the US legally from a country that is part of the third world, they often have imperfect paperwork, and lack of any real records. This means if they have been involved in criminal activity or violent behaviour, we may not even understand it, and surely if so, it is doubtful that they would undertake such information, and thus, be denied US Citizenship.
Part of what our immigration laws are envisioned for is to avert malcontents, evil doers, or bad people from joining us, but without good records, we could reject someone with minor offences in one nation that is of the first world with good records while permitting a murder to come here from the third world. You can also visit www.amity-law.com/immigration-lawyer-los-angeles to know more about the immigration laws.
Currently, there is loads of work being done in Europe to harmonize the criminal records and make them available to specialists in other nations, so fugitives can be caught, and other nations can shield against bad people, child molesters, murderers, and fraudulent business people who try to colonize to other nations, and we too get the assistances of the reciprocal.
Unfortunately, when it comes to third world nations which do not have decent records there is literally no hope. Worse, in many third world nations, those who oppose the governments, there are actually good people but have been thrown in jail or prison, because the people in charge have used the jails and law to silence opponents. This also hurts good people and it hurts us while we try to sort it all out.