Land disputes are as ancient as mankind, but the notion of national land is relatively new in human history. In prehistoric days, emotional attachments to "place" were more personal and sacred than abstract and legalistic. Yet as the most recent conflict in Israel and Palestine reminds us, people are more than willing to shed blood over a parcel of desert.

Why is that? What is it about a particular plot of land that stirs such deep passions in us? One emerging theory is that emotional ties to a specific place are a kind of magical thinking, an ancient belief in the power of contagion. Eons before modern germ theory, our ancestors came to believe that "essences" were passed on through physical contact. This was an adaptive belief back then, because it offered some protection against infection and so forth, but these taboos are often irrational when expressed today. Yet they are hard to shake because they're deep-wired into our neurons.

At least that's the theory, and there is considerable laboratory research to support the idea that humans imbue objects or land with positive or negative associations—whether it's rational or not. For example, people express a deep aversion to wearing a sweater worn by Adolf Hitler, even though that's completely irrational. Or, they refuse to eat food that's been touched by a cockroach, even if the cockroach has been sterilized in a lab setting. People also believe in positive essences—wearing a revered grandmother's ring, for instance—but bad essences usually trump the good ones. So, for example, Mother Teresa probably couldn't have decontaminated Hitler's sweater by wearing it; for most people, it's permanently tainted.

Two University of Pennsylvania scientists decided to explore the role of contagion beliefs in the ongoing land disputes of the Middle East, specifically the land of Israel. They figured that land, at least as much as jewelry or clothing, could be perceived as having either good or bad essences. Indeed, ancestral Jewish land could be imbued with associations so powerful that they would make trading or forfeiting the land taboo—even for an equivalent plot of land elsewhere. Enemy-occupied land would similarly be imbued with negative essences, thus creating a fundamental psychological conflict over the land of Israel. How does this primeval cognitive battle play out in the modern Jewish mind?

As part of this research, psychologists Paul Rozin and Sharon Wolf devised a laboratory tool to measure peoples' propensity for positive and negative contagion beliefs, and they used it to study both Israelis and American Jews, surveying a total of 318 subjects. They also asked them a variety of specific questions about their attachment to the land of Israel: Would they trade any part of East Jerusalem? The Temple Mount? An unoccupied parcel of Israeli land? And if so, to whom? Syria? Jordan?

Both Israelis and American Jews considered the land of Israel "untradeable." This was true not only for sacred sites like East Jerusalem, but also for unnamed parcels of border property. When asked about Har Herzl, a burial ground for the Zionist leader Teodor Herzl and assassinated Prime Minister Itzak Rabin, fully 83 percent of Israelis and 70 percent of American Jews said they "would never trade it for land, or anything else." This basically puts the land of Israel in a category with one's children or one's religion—completely off the table.