and 'what you got' with the approximate success rate of 85% for boys and 82% for girls. A bibliography of 129 references cites 28 papers of my own. The book has proven sound through the past 27 years; and, like the house in the scriptures, built on the rock rather than the sand.” There is some disagreement among researchers as to how and whether the method works. For example, the 1995 article "Timing of Sexual Intercourse in Relation to Ovulation—Effects on the Probability of Conception, Survival of the Pregnancy, and Sex of the Baby" in the New England Journal of Medicine concludes that "for practical purposes, the timing of sexual intercourse in relation to ovulation has no influence on the sex of the baby." The study tracked women's hormone levels daily to determine the day of ovulation. In contrast, another study from the New England Journal of Medicine (1979) found that "[our] results ... demonstrate that insemination on different days of the menstrual cycle does lead to variations in sex ratio." While this second study was larger, it did not track the levels of LH in the blood but instead used cycle lengths to determine probable time of ovulation. Adding support to another aspect of the Shettles's findings related to sperm morphology, an important element in his method, a 1997 study, "Size Differences Between X and Y Spermatozoa and Prefertilization Diagnosis," published in Molecular Human Reproduction, using polymerase chain reaction-aided techniques, concluded: “Statistically, the length, perimeter and area of the sperm heads , and the length of the sperm necks and tails of X-bearing spermatozoa were significantly larger and longer than those of Y-bearing spermatozoa" Multiple studies have confirmed Dr. Shettles's findings that the larger X-bearing, female producing spermatozoa are hardier, another factor that is important in his method. Men who work at high altitudes, in high heat environments and in situations that expose them to various toxic substances and other forms of physiological stress have been reported to give birth to far more girls than boys. OBGYN News, October 15–31, 1982, for example, reported on a study of undersea divers in Australia showing that their offspring were almost 2-1 female. The researchers cited other data indicating a similar situation involving pilots of high-altitude military aircraft. Dr. Shettles contended that atmospheric pressures, tight fitting clothing and many other stressors could favor the X-bearing spermatozoa. Another study, reported upon in Discover