In this cohort, childhood IQ and achieved education level were both positively associated with increased odds of upward mobility between the ages of 5 and 49-51 years. However, only education level was significantly associated with upward mobility between 5 and 25 years and only childhood IQ was significantly associated with upward mobility between 25 and 49-51 years. Lower childhood IQ was associated with increased odds of downward mobility.
As previously noted in studies of social mobility and health in this cohort [4, 13, 17], most mobility was in an upward direction. This reflects changes in occupational social class patterns found throughout the UK , where there was a considerable amount of absolute intergenerational social mobility over this period. Economic and social change meant that there was 'more room at the top' . For the Newcastle Thousand Families birth cohort, all born in 1947 in a city in the north of England , their career and educational opportunities are likely to have been far greater than that experienced by their parents as, although manufacturing has declined, there has been a massive growth in the service sector leading to a much larger middle class and the size of the unskilled manual labour force has decreased.
In this study, higher achieved education level was a significant driver of upward mobility. Education has been described as 'the main engine of social mobility'  and as a means to 'escape from childhood disadvantage' . Education has been associated with social mobility in men in a number of previous studies [9, 10, 12], measured either as number of years of education or as level of education, but only one of these studies also measured IQ. In that study high IQ was associated with upward mobility at the univariable level, but the association was lost in the multivariable model when education, height and number of siblings were included . However, men who were in an advantaged social class in adulthood had the highest childhood IQ scores, irrespective of their fathers' social class, suggesting that IQ may be important for upward social mobility . In the current study, whilst higher childhood IQ and education level were significant predictors of upward mobility between 5 and 49-51 years at the univariable level in men, only IQ was significant in the adjusted model, although this may have been due to the low numbers of men investigated.
High childhood IQ was significantly associated with upward mobility between ages 5 and 49-51 years and between 25 and 49-51 years in this cohort. Our findings in relation to childhood IQ and social mobility support previous evidence suggesting that IQ level is more strongly associated with social class attained in middle-age rather than early adulthood and that people with high IQs eventually move up the occupational ladder regardless of their educational qualifications . However, education does appear to be important for early mobility as only education level was significant for early social mobility between the ages of 5 and 25 years. As in previous studies , IQ and educational achievement were correlated. Higher childhood IQ has been associated with higher age of leaving education for men  who are therefore likely to obtain higher level qualifications, but this may also depend on the social circumstances of the family.
In a meritocracy, individuals should be able to improve their circumstances through ability and effort. There is a split in the literature between those who believe that the UK is 'to a large extent a meritocratic society'  where 'IQ plus effort' can allow someone to advance up the social scale, and those who argue that children from more disadvantaged origins 'need to show substantially more merit'  in order to reach the same levels. Results from this study suggest that higher IQ in childhood is associated with upward mobility, supporting the meritocracy theory that 'IQ plus effort' does indeed allow upward mobility. However, in the descriptive analyses the highest IQ levels were found in the stable non-manual group rather than the upwardly mobile, a pattern that has been reported elsewhere . Therefore, it appears that those from more disadvantaged backgrounds in this study do not need to exhibit 'more merit' than those who originate from a higher social class, in order to reach the same broad level on the social scale. However, we cannot say anything about finer stratification within each occupational social class, where evidence still suggests that those from more privileged backgrounds tend to occupy the more lucrative and prestigious professions .
Low childhood IQ  and low achieved education levels [10, 12] have previously been associated with downward mobility in men, although only one study considered both . In the current study, those with a lower childhood IQ were significantly less likely to be upwardly mobile and more likely to be downwardly mobile, suggesting that low childhood IQ is both a barrier to upward mobility and a driver of downward mobility. Education level was not significantly associated with downward mobility.
Previous studies have largely focused on men, and social mobility in women has not been explored. In this study childhood IQ was significantly associated with upward mobility between 5 and 49-51 years and between 25 and 49-51 years for women. University education also appeared to be important for women in achieving upward social mobility as all women in this cohort who had a university-level education were upwardly mobile. Education level, but not childhood IQ, was significant for upward mobility in women between ages 5 and 25 years.
It was found that, for each level of education, women had a higher median IQ than men. Therefore it appears that women in this study, who were as intelligent as men, did not achieve the same level of education. Members of this cohort were teenagers in the 1960s when only a small percentage of the population went to university and there were still high levels of inequality between the sexes. It is possible that the observed sex differences are due to the social values of the time, where education was not considered as important for women. However, for those women who did go on to higher education this allowed them to progress up the social scale or maintain their high social position, as university degree-level or post-graduate qualifications were only found in stable non-manual or upwardly mobile women.
It has been suggested that educational success might not necessarily result in upward mobility if many disadvantages have been suffered in childhood . However, very few data are available on the influence of childhood social circumstances on social mobility. In this current study where it was possible to examine this, adverse circumstances (including parental debt, death, divorce and imprisonment) and poor housing conditions in childhood did not significantly affect the chances of social mobility in either direction.
In children who experience deprivation, those who are most advantaged in terms of social capital are more likely to be upwardly mobile . Social capital can be measured in material, psychological and cultural terms . Conditions in childhood which result in low levels of social capital might also contribute to shorter stature  so that height can be used as a proxy measure of childhood circumstances as inadequate nutrition contributes to attained adult height . Achieved adult height has been directly linked to social mobility in men  with taller men who were born between 1905 and 1935  and who were born in 1921  showing upward mobility, with downward mobility seen for those who were shorter. In the current study, with participants born in 1947, height was not significantly associated with social mobility. There is a lack of published data relating to height and social mobility in women. Here, no association was found between height and social mobility in women. It is possible that due to the post-war conditions that the cohort grew up in, with rationing still in place until 1954, potential height differences by social class were reduced, resulting in non-significant differences in height between socioeconomic trajectories, as found here for both men and women.
The main strength of this study is the prospective nature of the data and the extensive follow up covering 50 years, which allows analysis of both inter-generational and intra-generational social mobility. The study examined social mobility in women as well as men, something that has not been done before, although numbers were low when the sexes were examined separately. Some retrospective data were obtained to determine social class at age 25 years, but all childhood and age 50 data were prospectively collected thus reducing the possibility of recall bias. The wide ranging available data meant that the indirect selective effects of social, biological and educational factors on social mobility could all be examined simultaneously. However, whilst the range of data examined allowed confounding to be taken into account, the possibility of residual confounding having a role in interpreting the findings remains.
As data on social factors such as adverse circumstances and housing conditions in childhood were available, it was possible to examine whether those who lived in more deprived circumstances achieved their educational potential and whether this influenced social mobility. IQ data were collected prior to educational attainment and the influence of both was evaluated, which allowed consideration of whether those from lower social class backgrounds need to show 'more merit' than those from more privileged households in order to obtain similar adult socio-economic standing. As the cohort members were aged 49-51 years at the time of data collection for this study, it was possible to collect details of educational attainment achieved later in life. Social class at age 5 years rather than at birth was used. With the UK recovering from war in 1947, many of the occupations at the time may not have truly represented the head of household's true standing. Taking social at age 5 years is likely to give a more accurate picture of the individual's childhood socio-economic circumstances than using birth data.
Loss to follow-up can be an issue in longitudinal cohort studies and is not always randomly distributed. Here, there was no significant difference in childhood social class between those included in this study and the remainder of the cohort. A significantly higher proportion of women were included in this study than in the remainder of the cohort, but the sexes were examined separately.
The problems inherent in assigning a SES based on occupation to women should also be acknowledged, particularly when using the highest occupational social class in the household which, for many women in this cohort, resulted in them being assigned the SES of their male partner. However, the use of husband's occupation to determine social class for women can also be regarded as a strength, as it is debateable how relevant a women's own occupation actually is as a reflection of socio-economic status, as this tends to be compromised by motherhood . Becoming a mother can often result in apparent downward social mobility for women  as they move from an employed to a caring role. We did not have data on the number of women in this cohort for whom this was the case, but it is likely that many of these women did not work or gave up work when they had children . Therefore the husband/partner's SES is more likely to truly reflect the SES of the household.
Those who remained in the study had significantly higher median childhood IQ than those who did not. Low childhood IQ in this cohort has been associated with an increased risk of early mortality , which might partially explain why those who remained had higher IQ. Low IQ has also previously been associated with increased risk of morbidity  and with adverse health behaviours .
Although this study was able to include some adverse social and material circumstances in childhood, the fact that these markers of deprivation and adversity were not associated with social mobility in this cohort might suggest that other measures of social support that we were not able to determine may play a role in limiting some of the adverse effects of socio-economic disadvantage in childhood. This could be an area for future research in this cohort.