If you have anything beyond a cursory interest in the Zodiac Killer, you probably know that he created a cryptogram in which he purported to encipher his name. This cipher is often referred as the My Name Is cipher because that's the phrase the killer used to introduce it. Alternatively, people sometimes refer to it as the 13 cipher, a reference to the number of symbol instances.
A little-known and infrequently-discussed part of this cipher is the probable motivation that the killer had for creating it, namely a particular newspaper article in the Examiner. There is a passing, indirect reference to the story in Robert Graysmith's Zodiac , but one that neither develops the proper association with the cipher nor explores all aspects of its relevance.
The pinnacle of the Zodiac's impact on the San Francisco Bay Area occurred in the aftermath of taxicab driver Paul Stine's murder. A near panic resulted from the combination of the killer: (a) striking in the city itself (as opposed to the North Bay), (b) becoming more unpredictable as evidenced by his willingness to break previously established behavioral patterns such as targeting only couples, and (c) threatening to kill school children.
Predictably, the massive level of concern over the elusive serial killer resulted in deluge of media attention. For approximately a week just after the middle of October in 1969, both the Chronicle and the Examiner published a constant stream of stories about the Zodiac, sometimes multiple stories per day.
It is within this time frame that we find the nearly certain source of motivation for the My Name Is cipher. On October 22, 1969, the Examiner published three separate stories about the Zodiac. Being the day after the infamous Jim Dunbar Show incident, there was an obligatory story covering that fiasco . Another, rather well-known story  documented Joe Stine's (Paul's brother) message to the Zodiac. Interestingly, the sibling published his daily work routine and challenged the killer to come find him.
The article of interest, however, wasn't one of these front page stories; rather, it was an article relegated to page nine . Entitled Cipher Expert Dares Zodiac To 'Tell' Name, the story described "a challenge perhaps unique in the annals of American crime...." Specifically, the president of the American Cryptogram Association (ACA), Professor D.C.B. Marsh — who had previously validated the Harden's solution to the 408 cipher — dared the Zodiac to construct a cipher that legitimately enciphered his name. The article included the following quotes from Dr. Marsh:
The killer wouldn't dare ... as he has claimed in letters to the newspapers ... to reveal his name in a cipher to established cryptogram experts.
I invite 'Zodiac' to send to The American Cryptogram Association, care of Dr. D.C.B. Marsh. president. ... a cipher code — however complicated which will truly and honestly include his name — for study by myself and my colleagues in the association.
The relevance of this dare is patently obvious. Marsh publicly challenges the Zodiac to create a cryptogram in which he honestly enciphers his name, and the killer constructs a cipher that he introduces via the phrase "My name is."
Clearly, the Zodiac did not send the cryptogram to Marsh, as had been requested (this non-public way of satisfying the challenge was apparently unacceptable to the attention-craving serial killer). Neither did he respond in a time frame that most would have expected. But these details matter little. In fact, the timing itself provides yet another reason to conclude that what we have here is a specific instance of cause and effect. As mentioned, this article was published in the Examiner on October 22, 1969. Six months to the day later, April 22, 1970, the people of the San Francisco Bay Area were reading about the My Name Is cipher in the Chronicle .
This relationship between the Examiner article and the My Name Is cipher may well mean there is an even higher likelihood that the cryptogram does indeed encipher some form of the killer's name; the reasoning being that the Zodiac would have felt compelled to abide by the rules of the challenge in order to legitimately win the dare and thereby prove his intellectual superiority to Dr. Marsh, the ACA membership, and the rest of the cipher-solving world.
 Robert Graysmith. Zodiac. New York: Berkley Books, 1987. 122.
 Hubert J. Bernhard. “TV 'Zodiac' Reneges on Surrender.” San Francisco Examiner (Oct. 22, 1969), 1.
 “A Challenge to the Zodiac.” San Francisco Examiner (Oct. 22, 1969), 1.
 Will Stevens. “Cipher Expert Dares Zodiac To ‘Tell’ Name.” San Francisco Examiner (Oct. 22, 1969), 9.
 Paul Avery. “Zodiac Sends New Letter–Claims Ten.” San Francisco Chronicle (Apr. 22, 1970), 1.