three techniques: anonymous respondents are provided with a checklist of fantasies and asking them to indicate which ones they have experienced, how often, and in what context. This method relies on retrospective recall, which may limit its accuracy. A checklist may not be comprehensive, and as a result may be biased towards some fantasies. anonymous respondents are asked to write, in narrative form, their sexual fantasies. This method also relies on retrospective recall. Some studies limit the number of fantasies entered (such as only the most frequent ones), and respondents may not write down all of their fantasies anyway-—they may forget infrequent fantasies, not want to write too many down, or be more subject to social desirability bias than with a checklist. respondents record the fantasies they experience over a given period of time using checklists or diaries. This method requires a long period of time to be representative, and may be impractical. To measure the reliability of a person's reporting of fantasies, researchers may compare a person's reported sexual arousal against actual measures of arousal, using techniques such as vaginal photoplethysmography, penile strain gauges, or other tools, such as genital pulse amplitude, genital blood volume, and heart rate. A 1977 study found that males judged arousal based on blood volume far better than females, and that males and females were equal when judging arousal based on pulse amplitude measures. Additionally, females were better at judging low arousal. As with studies of sex in general, samples used in studies may be too small, not be fully random, or not fully representative of a population. This makes similarities between studies especially important. Women may be prone to underreporting the frequency of fantasy because they do not realize that they are becoming aroused, or they will not say that they are; one common problem is that they will imagine romantic imagery and become aroused, but not report the fantasy because it is not sexually explicit. Many studies are modern and are carried out in western society, which, through factors like gender roles and taboo, are not widely representative, raising the need for more studies in different societies and historical eras. With regards to age, there is very little knowledge of sexual fantasies in children aged 5 to 12, and there is a need for longitudinal studies across a life span. Sex is often a taboo topic, so conducting a truly honest and