Summary 1. The phenomenon of frequency‐dependent selection with an advantage for the rare type over the common type is intriguing because it implies balancing selection. Thus the high level of genetic variability as found in natural populations can be explained without the necessity of considerable genetic load. Rare male mating advantage is here defined as frequency‐dependent male sexual fitness with the rare type of male favoured. Such a rare male effect has been found to be very widespread in insects, at least under laboratory conditions, but there are several problems associated with this phenomenon which will be discussed in this review. 2. To determine whether male mating success is frequency‐dependent, the quantity to be considered, most appropriately, is male sexual fitness of the one type relative to the other type (KM). Other approaches are discussed and it is shown that they confound differential mating success with frequency dependence of mating success. Moreover, it is shown that Levene's indices, previously designed as a measure of differential mating success, confound mating success with assortment, making these indices less useful. 3. The theoretical relationship between frequency dependence of male mating success and total sexual fitness is more complicated than would be expected beforehand. Some examples are given to clarify this issue. 4. Statistical tests to determine frequency dependence of male mating success have often been carried out in the past by determining the significance of deviations from random mating for each male type frequency separately. This procedure must be considered incorrect, because a change in male mating success over frequencies has to be tested. A correct way to do this is by testing all frequencies together in one single statistic, with only a moderate assumption about the type of frequency dependence. When mating success depends on frequency in a more irregular way, alternative tests are available, in which mating success at one frequency can be tested against any other frequency. 5. The rare male effect has been studied most thoroughly in Drosophila, and has been demonstrated for many Drosophila species. The effect has been demonstrated for some other insects as well, and also for vertebrates. The rare male effect has been found for types of males differing in specific genotype (visible mutants, karyotypes), genetic background and geographic origin. A rare male effect has also been demonstrated for non‐genetic properties such as temperature of rearing. Though much less common than rare male mating advantage, there are some examples of rare female mating advantage. The expression of the rare male effect may be affected by several factors, such as age of the females, temperature or experimental approach. 6. Only a few studies on rare type mating advantage in Nature have been carried out, but some positive evidence is available. 7. It is pointed out that mating success will be frequency‐dependent if both types of males differ pronouncedly in mating behaviour, but the...

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Multiple choice mating experiments were carried out with the alcohol dehydrogenase (Adh) genotypes FF and SS of Drosophila melanogaster. Rearing conditions and age of the flies were varied. The large mating advantage of FF for 4-6 days old flies, reared at 25°C, found by Pot et al. (1980), was confirmed. This advantage decreased or even disappeared when rearing temperature or age of the flies was lowered. The relative humidity during rearing also influenced mating success. The relevance of these results for the maintenance of the Adh polymorphism is discussed.

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