My degree is from your department (thesis advisor George Field) '77. Philip Morrison, MIT, of "Search for Interstellar Communications" fame was my undergrad advisor. I remember Donald Menzel (Project Blue Book) roaming the Observatory halls. I think back half a century and the biggest change in consideration of ETI is with the robust identification of 5000+ exoplanets. We are likely the newest and thus dumbest civilization in the galaxy, for any reasonable parameters in the Drake equation (now informed by the ubiquity of expolanets); as you know the civilization (defined as radio technology possessing) lifetime L is likely the most uncertain parameter, we have one sample L ~ 100 years. In which case we are the dumbest if L actually > 1000s (why shouldn't it be, modern humans date back a couple 100,000 years). Being the dumbest suggests caution (mostly passive observation) and humility. But if they are nearby and much more advanced we cannot really hide. We are fortunate that the interstellar travel distances are likely very long relative to lifetimes since our one experience of intelligent life is of a predatory and expansion-oriented species, as apex species tend to be. The next half century should be very telling in terms of the search for solar system primitive life, possible relics of other civilizations therein, and the signatures of life in atmospheres of exoplanets. Looking forward to reading your new book.