4.1 Rarity of intelligent life

    4.1.1 Extraterrestrial life is rare or non-existent

    4.1.2 Extraterrestrial intelligence is rare or non-existent

    4.1.3 Periodic extinction by natural events

4.2 Evolutionary explanations

    4.2.1 Intelligent alien species haven't developed advanced technologies

    4.2.2 It is the nature of intelligent life to destroy itself

    4.2.3 It is the nature of intelligent life to destroy others

    4.2.4 Civilizations only broadcast detectable signals for a brief period of time

    4.2.5 Alien life may be too alien

4.3 Sociological explanations

    4.3.1 Colonization is not the cosmic norm

    4.3.2 Alien species may have only settled part of the galaxy

    4.3.3 Alien species may not live on planets

    4.3.4 Alien species may isolate themselves from the outside world

4.4 Economic explanations

    4.4.1 Lack of resources needed to physically spread throughout the galaxy

    4.4.2 It is cheaper to transfer information than explore physically

4.5 Discovery of extraterrestrial life is too difficult

    4.5.1 We haven't listened properly

    4.5.2 We haven't listened for long enough

    4.5.3 Intelligent life may be too far away

    4.5.4 Intelligent life may exist hidden from view

4.6 Willingness to communicate

    4.6.1 Everyone is listening but no one is transmitting

    4.6.2 Communication is dangerous

    4.6.3 Earth is deliberately avoided

    4.6.4 Earth is deliberately isolated (planetarium hypothesis)

4.7 Alien life is already here unacknowledged