For example,they do not compensate for gross social inequality,and thus do not tell how able an underprivileged youngster might have been had he grown up under more favorable circumstances. 这句话里怎么have been 后又来个had he grown up啊？这算什么意思啊？顺带翻译一下？

For example,they do not compensate for gross social inequality,and thus do not tell how able an underprivileged youngster might have been had he grown up under more favorable circumstances.

例如，测试并不能解决严重的社会不公问题，且不能判定一来自社会底层的年轻人，如果有更好的成长环境，会有多大才干。

这里had he grown up under more favorable circumstances是省略if，将助动词had提前倒装的，虚拟语气的条件从句。只是直接和主句连一起让你产生疑惑。如果之前有个逗号，就会很明显。估计是考试为了增加迷惑性，故意将原文前的逗号去掉。