Gates and Gaps Together
It is particularly important to understand how gates and gaps interact over time to produce gates and gaps at the next stage.  A good example is provided by individuals applying to college at the end of high school. Students from poorer families with weak grades and low test scores face many closed gates: Not only are top-tier colleges and universities out or reach, but when they enroll at community colleges they find that they must take one or more so-called remedial courses, a path that often leads to dropping out before obtaining a degree or certificate. (Bettinger et al. 2013; National Center for Public Policy and Higher Education Special Report 2010). 
Sometimes the gates are less obvious. Students coming from families without college experience are less adept at navigating the admissions and financial aid processes and have fewer resources upon which to draw. In fact, a recent study finds that many able, top-scoring minority students coming from lower SES families don't even apply to top-tier colleges, thinking they don't qualify and couldn't afford them if they were accepted (Hoxby and Avery 2013). This problem stems from the lack of a certain kind of social capital, a lack that is amenable to policy intervention (Hoxby and Turner 2014).
One consequence of this dynamic between gates and gaps is relatively homogeneous college campuses, leading to assortative mating and further divergence in personal/family trajectories (McClanahan 2014). This divergence is even more pronounced when one looks at the full birth cohort, which includes those who dropped out of high school or completed high school but did not go on for further education or training (see Chap. 7, Fig. 7.13).
Gradients is a metaphor for the strength of the relationships between the dimensions of human capital on the one hand, and various life outcomes on the other. Life outcomes include whether the individual is employed, the nature and remuneration (salary and benefits) of that employment, the possibility of obtaining further education/training, accumulation of wealth, the likelihood of forming stable family units, and having children in the context of those partnerships. The data show that the gradients are typically quite steep; that is, modest differences in human capital can result in substantial differences in outcomes. For example, both the likelihood of full-time employment and the likelihood of having children in the context of a twoparent family are strongly correlated with levels of educational attainment and cognitive skills. Gradients are critical because they account for much of the relative advantage or disadvantage that is passed on to the next generation.
It is worth pointing out that gradients are typically not linear. That is, there are inflection points such that there can be large differences in outcomes for individuals who are close in many facets of human capital. For example, individuals with similar cognitive skills but who differ in whether they obtained a college degree can have very different adult trajectories. On the other hand, differences between inflection points may be less consequential.
In the remainder of the chapter, the gates/gaps/gradients framework will be used to organize some of the voluminous literature concerning the forces and processes driving differences in opportunity, as well as the extent of those differences.
-  The recognition that such dynamic interactions over, say, ages 0–18 can have powerful effects on the distribution of adult skills is implicit in Brookings Institution, Social Genome Project, and explicit in the work of James Heckman and his associates heckmanequation.org/content/ resource/case-investing-disadvantaged-young-children.
-  The problem of high school graduates going on to tertiary education but required to take one or more noncredit-bearing courses (sometimes designated as remedial or developmental) is more pervasive than one that just concerns students from low-income families. While some studies estimate 35–40 % of students entering college need at least one remedial course, other studies place the estimate as high as 60 %.