No one can declare with an absolute degree of certainty that Theodore Kaczynski committed the crimes attributed to the Zodiac. There exists, however, an undeniable inference that the sheer weight and number of the similarities argue for a genuine connection between the two. The cogence of the similarities cries out for a forthright answer to the question "was Theodore Kaczynski the Zodiac Killer?"
Though the questions are not amenable to a definite answer, one may reach a satisfactory and logical conclusion based on the probabilities inherent in the comparisons. A recapitulation of the evidence shows that both Kaczynski and the Zodiac are "disaffected" killers, driven by sexual frustration and the need to take symbolic vengeance. Each uses the notoriety achieved by murder to seek publicity on a national scale. Each forwards taunting letters to the media demanding publication on pain of further killings. Each places taunting calls to the police, while each creates an outdoor graffiti consisting of crossed lines within a circle. Each mails a letter to the editor of the San Francisco Chronicle threatening mass murder, then issues a wryly-worded retraction once panicked authorities have given the appropriate response. Both moved in key areas during several occurrences relating to Zodiac events. Their descriptions are strikingly similar, particularly regarding their heavy facial structure and prominent lower jaw. Both display intellectual versatility and show bomb-making skills approximately at the same level in 1969. Both employed elaborate cryptosystems, while both used literary allusion to cast hints regarding their whereabouts and motives. Both show numerous similarities in compositional tone and style, which practically mirror one another. The handprinting style of Kacyznski may easily be disguised to form the hand of Zodiac.
Statistically, the probability that any two individuals share the qualities listed above is very small. To calculate such a probability, one would have to begin with the relatively tiny population of known murderers, estimated at roughly .0089 percent of the entire population, or around 19,669 individuals out of the 221 million people living in the U.S. in 1969. From this figure must be deducted all females, all non-Caucasians, all those not in the age ranges of 2545, all those not classified as "disaffected killers" (mass murder types), and all those not using murder as a vehicle for national publicity. Having done so, one will arrive at a figure of .62 (sixty-two one-hundredths) of an individual out of the original 221 million that was started with. That is, however, before accounting for specific, hard-to-quantify factors such as intelligence quotient, bomb-making, cryptography, mathematical abilities, simliarities in appearance, handwriting, and the lengthy list of quirks in style and tone that are manifested by Kaczynski and the Zodiac. One must also account for the existence of the line-in-circle graffiti left by both killers in conjunction with their crimes, and both killers' use of literary allusion, a very rare quality indeed.
In the final analysis, the likelihood of any one person possessing all the qualities that identify both of these famous killers can be conservatively estimated at billions to one. And while this may not be tantamount to a "smoking gun," it nonetheless speaks volumes in terms of providing objective support to what comprises a very good subjective case for Kaczynski as a suspect.
A detailed picture of the similarities is presented in The Unabomber and the Zodiac, by Zodiologist and Kaczynski theorist Douglas Evander Oswell. This work is now available in standard book format.