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Parasitic wasps propagate by injecting their eggs into a caterpillar that then becomes paralyzed as the eggs inside develop into wasp larvae. The wasp larvae kill the caterpillar host as they feed on it, form cocoons, and finally develop into wasps. In attempting to discover how such wasps detect the presence of the caterpillar hosts that are so critical to the wasps' propagation, researchers have uncovered an intriguing defense mechanism developed by the plants on which the caterpillars feed.

When chewed on, many plants release volatile compounds from both damaged and undamaged tissues. When these compounds are toxic to the insects that feed on the plants, they can help defend the plants from such attacks. However, the plants on which the wasps' caterpillar hosts feed have evolved an even more complex defense: the caterpillar-infested plants appear to release volatile chemicals that attract parasitic wasps, which then prey on the caterpillars. Scientists originally suspected that the wasps were attracted by an odor, reminiscent of cut grass, that is released as the caterpillar feeds, but a recent study suggests that a different set of volatile attractants is involved. In this study, when researchers used a razor blade to mimic caterpillar damage on the leaves, only grassy odors were emitted, not the volatile compounds that attracted wasps. However, when oral secretions from the caterpillars were applied to these damaged leaves, the leaves released the wasp attractants several hours later. Further tests revealed that oral secretions placed on the razor-damaged leaves stimulated the release of such attractants, making the plants as attractive to wasps as plants that had suffered actual caterpillar damage. These results suggest that chemicals from the caterpillar must be present for these attractants to be released and that unlike the grassy scent, which emanates only as the caterpillar on the plant, the wasp attractants are produced several hours after the attack and persist for several hours, perhaps days. Researchers have launched additional studies to determine whether the wasps' capacity to prey on caterpillars can be enhanced to the extent that the wasps could be used as a natural pesticide to "police" plants and protect them from crop-destroying caterpillars.

Mathematical principles of probability entail that for any future event, the probability that it will occur Is at least as great as the probability that both it and some other given event will occur. Consider, for example, the following statements that were shown to subjects in a 1998 study.

X The percentage of adolescent smokers In Texas will decrease at least 15% from current levels by September 1, 1999.

Y The cigarette tax in Texas will increase by $1.00 per pack in 1999.

Z The cigarette tax in Texas will increase by $1.00 per pack in 1999, and the percentage of adolescent smokers in Texas will decrease at least 15% from current levels by September 1, 1999.

Z("Kand X") could not have been more probable than X. Nevertheless, many of the subjects judged Zto be more probable than X. This mistaken form of reasoning, displayed with surprising frequency in various studies in addition to the 1998 study, is known as the "conjunction fallacy."

A number of researchers have offered alternative explanations for the seeming manifestations of the mistake, thus arguing that the fallacy is less widely committed than the various studies would indicate. Some have claimed that research subjects can take "probability" in a sense that does not conform to the mathematical principles of probability. Detailed descriptions of some such conceptions of "probability" have been developed under the names of "confirmation" and "support." Other researchers would claim, correctly, that subjects shown Z(" Kand X") and ^simultaneously will sometimes think of Xas involving the negation of Y—as a claim that the percentage of adolescent smokers in Texas will decrease, but without the $1.00 increase in the cigarette tax.

However, although the subjects in the 1998 study were to consider Xand Z simultaneously, the statements were presented in terms of bets rather than explicit requests for judgments of relative probability. Subjects were asked to choose between Zand X, with a chance of winning $50.00 if the chosen statement turned out to be true. Terms such as "most probable," "likely," etc., were thus avoided, and the interpretation of X\n conjunction with the negation of Kwas thereby eliminated. And with these alternative explanations eliminated, many of the subjects nonetheless bet on Zrather than X: