INCREASES IN THE PROBABILITY OF APPREHENSION, THE CONDITIONAL PROBABILITY OF INCARCERATION, AND THE SENTENCE ARE ASSOCIATED WITH SIGNIFICANT REDUCTIONS IN AIRCRAFT HIJACKINGS IN THE 1961-76 TIME PERIOD. THESE FINDINGS ARE BASED ON TWO METHODS OF ESTIMATING THE RATE OF HIJACKINGS--A QUARTERLY TIME SERIES AND THE TIME OR FLIGHT INTERVALS BETWEEN SUCCESSIVE HIJACKINGS AND ALTERNATIVE ESTIMATES OF THE DETERRENCE VARIABLES. TO TEST AN ALTERNATIVE EXPLANATION OF HIJACKING, TERMED THE 'FAD' HYPOTHESIS, FOREIGN HIJACKINGS ARE INCLUDED AS AN INDEPENDENT VARIABLE IN REGRESSIONS ON U.S. HIJACKING. SINCE THE NUMBER OF FOREIGN HIJACKINGS COINCIDE WITH VARIATIONS IN THE INTENSITY OF THE WORLDWIDE HIJACKING FAD, THE INCLUSION OF THIS VARIABLE ALLOWS THE DIFFERENTIATION BETWEEN DETERRENCE AND FAD EFFECTS. ALTHOUGH FOREIGN AND U.S. HIJACKINGS ARE POSITIVELY CORRELATED, THE DETERRENCE VARIABLES REMAIN HIGHLY SIGNIFICANT AND APPEAR TO BE THE RELATIVELY MORE IMPORTANT DETERMINANTS OF U.S. HIJACKINGS. REGRESSION ESTIMATES FROM THE SAMPLE PERIOD ENDING IN 1972 WERE USED TO FORECAST THE NUMBER OF ADDITIONAL HIJACKINGS THAT WOULD HAVE TAKEN PLACE BETWEEN 1973 AND 1976 IF MANDATORY SCREENING HAD NOT BEEN INSTITUTED AND THE PROBABILITY OF APPREHENSION HAD REMAINED CONSTANT AND EQUAL TO ITS 1972 VALUE. UNDER THESE ASSUMPTIONS, THERE WOULD HAVE BEEN BETWEEN 41 AND 67 ADDITIONAL HIJACKINGS COMPARED TO THE 11 THAT ACTUALLY OCCURRED BETWEEN 1973 AND 1976. ALTHOUGH THE MANDATORY SCREENING PROGRAM IS HIGHLY EFFECTIVE IN TERMS OF THE NUMBER OF HIJACKINGS PREVENTED, ITS COSTS ARE ENORMOUS. THE ESTIMATED NET INCREASE IN SECURITY COSTS DUE TO THE SCREENING PROGRAM (WHICH DOES NOT INCLUDE THE TIME AND INCONVENIENCE COSTS TO PERSONS SEARCHED) IS $194.24 MILLION OVER THE 1973-76 PERIOD. THIS TRANSLATES INTO A $3.24 TO $9.25 MILLION EXPENDITURE TO DETER A SINGLE HIJACKING. AN APPENDIX CONTAINS AN EMPIRICAL ANALYSIS OF THE DETERMINANTS OF THE PROBABILITY OF APPREHENSION AND THE SEVERITY OF SANCTIONS. (AUTHOR ABSTRACT MODIFIED--RCB)