of their offspring, in such a way as to maximize their reproductive investment. Mate attractiveness and quality Similar to the idea behind the Trivers-Willard hypothesis, studies show that mate attractiveness and quality may also explain differences in sex ratios and offspring fitness. Weatherhead and Robertson (1979) predicted that females bias the sex ratio of their offspring in favor of sons if they are mated to more attractive and better quality males. This is related to Fisher's “sexy son” hypothesis, which suggests a causal link between male attractiveness and the quality of sons based on the inheritance of “good genes” that should improve the reproductive success of sons. Fawcett (2007) predicted that it is adaptive for females to adjust their sex ratio to favor sons in response to attractive males. Based on a computer model, he proposed that if sexual selection favors costly male traits, i.e. ornamentation, and costly female preferences, females should produce more male offspring when they mate with an attractive male compared to an unattractive male. Fawcett proposed that there is a direct correlation between female bias for male offspring and attractiveness of their mate. Computer simulations have costs and constraints, and selection may be weaker in natural populations than it was in Fawcett's study. While his results provide support for the Trivers-Willard hypothesis that animals adaptively adjust the sex ratio of offspring due to environmental variables, further empirical studies are needed to see if sex ratio is adjusted in response to mate attractiveness. Sex change The principles of the Trivers-Willard hypothesis can also be applied to sequentially hermaphroditic species, in which individuals undergo sex change. Ghiselin (1969) proposed that individuals change from one sex to another as they age and grow because larger body size provides a greater advantage to one sex than the other. For example, in the bluehead wrasse, the largest males have 40 times the mating success of smaller ones. Thus, as individuals age, they can maximize their mating success by changing from female to male. Removal of the largest males on a reef results in the largest females changing sex to male, supporting the hypothesis that competition for mating success drives sex change. Sex allocation in plants A great deal of research has focused on sex allocation in plants to predict when plants would be dioecious, simultaneous hermaphrodites, or demonstrate both in the same population or plant. Research has also examined how outcrossing,