...if taken seriously, have usually been considered signs of immaturity or lack of common sense. (Para. 2)
a sign of/signs of: something that shows the evidence of sth.
another sign of a person's nature (Para. 9)
A nod is a sign of agreement. 点头就是表示同意。
Words are the signs of ideas. 词语是思想的符号。
common sense: the ability to behave in a mature way and make practical decisions
Sometimes we guess the meanings of unknown words or expressions in a reading passage through common sense. 有时我们通过常识来猜阅读文章中的生词或词组的意义。