between paternal age and schizophrenia.; paternal age affects offspring's behavior, possibly via an epigenetic mechanism recruiting a transcriptional repressor rest. semen a 2001 review on variation in semen quality and fertility by male age concluded that older men had lower semen volume, lower sperm motility, and a decreased percent of normal sperm. a 2014 review indicated that increasing male age is associated with declines in many semen traits, including semen volume and percentage motility. however, this review also found that sperm concentration did not decline as male age increased. x-linked effects some classify the paternal age effect as one of two different types. one effect is directly related to advanced paternal age and autosomal mutations in the offspring. the other effect is an indirect effect in relation to mutations on the x chromosome which are passed to daughters who are then at risk for having sons with x-linked diseases. history birth defects were acknowledged in the children of older men and women even in antiquity. in book six of plato's republic, socrates states that men and women should have children in the "prime of their life" which is stated to be twenty in a woman and thirty in a man. he states that in his proposed society men should be forbidden to father children in their fifties and that the offspring of such unions should be considered "the offspring of darkness and strange lust." he suggests appropriate punishments be administered to the offenders and their offspring. in 1912, wilhelm weinberg, a german physician, was the first person to hypothesize that non-inherited cases of achondroplasia could be more common in last-born children than in children born earlier to the same set of parents. weinberg "made no distinction between paternal age, maternal age and birth order" in his hypothesis. in 1953, krooth used the term "paternal age effect" in the context of achondroplasia, but mistakenly thought the condition represented a maternal age effect.: 375 the paternal age effect for achondroplasia was described by lionel penrose in 1955. at a dna level, the paternal age effect was first reported in 1998 in routine paternity tests. scientific interest in paternal age effects is relevant because the average paternal age increased in countries such as the united kingdom, australia and germany, and because birth rates for fathers aged 30–54 years have risen between 1980 and 2006 in the united states. possible reasons for the increases in average paternal age include increasing life expectancy and increasing rates of divorce and remarriage. despite recent increases in average paternal age, however, the oldest father documented in the medical literature was born in 1840: george isaac hughes was 94 years old at the time of the birth of his son by his second wife, a 1935 article in the journal of the american medical association stated that his fertility "has been definitely and affirmatively checked up medically," and he fathered a daughter in 1936 at age 96. in 2012, two 96-year-old men, nanu ram jogi and ramjit raghav, both from india, claimed to have fathered children that year. medical assessment the american college of medical genetics recommends obstetric ultrasonography at 18–20 weeks gestation in cases of advanced paternal age to evaluate fetal development, but it notes that this procedure "is unlikely to detect many of the conditions of interest." they also note that there is no standard definition of advanced paternal age; it is commonly defined as age 40 or above, but the effect increases linearly with paternal age, rather than appearing at any particular age. according to a 2006 review, any adverse effects of advanced paternal age "should be weighed up against potential social advantages for children born to older fathers who are more likely to have progressed in their career and to have achieved financial security." geneticist james f. crow described mutations that have a direct visible effect on the child's health and also mutations that can be latent or have minor visible effects on the child's health; many such minor or latent mutations allow the child to reproduce, but cause more serious problems for grandchildren, great-grandchildren and later generations.