[SmG Lin]

They keep it a secret mainly to keep other people from worrying. Such as if everyone knew about something the government was doing, that could possibly destroy humanity as we know it. It can cause enemies of that government to get involved and do a weaponry race like there was back in the 60s with the US and the Russians. Also if everyone knew about everything, such as say aliens have been monitoring us this entire time and the government told everyone. It would cause a wave of people acting out in fear, which will cause riots and others questioning everything around them.

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Yes there may be some pros but you also have to look at the cons. In this situation, the cons weigh out the pros and in all, would not be a good idea.