Easy To Be Hard by Three Dog Night. From the Zodiac soundtrack.

David Fincher on choosing Easy To Be Hard as the opening song for Zodiac:

“It was odd ‘cuz I was in 2nd or 3rd grade when this whole thing happened [the Zodiac murders]. And I remember so vividly driving through Sonoma from Vallejo. It’s an area known as Black Point. And I remember being at the back of my parents’ ‘65 Impala, windows rolled down, it was the beginning of summer, and you can smell eucalyptus and this song was playing on the radio. There was something about it when I heard it. It transported me personally to the summer of 1969 which is, again, purely a subjective thing. And, you know, making movies ultimately is a wholly subjective thing. But I remember just feeling very misty when I heard that music playing over those pictures.”