I think that self-esteem plays a huge role in MacLeod's argument. Self-esteem (or lack thereof) comes from many different sources. A major source are parents or home life and another very important source is school and friends. It appears that the HH's have a lower self-esteem or sense of self-worth than the Brothers. Many of the parent of the HH's do not have high hopes for their sons, mainly because they do not want them to crash when they don't reach the top and several of the parents of the Brothers expressed the want for their sons to be successful. In my life, I had a step parent tell me that I would never go to college and never amount to anything, and I almost chose that path because that's what was expected of me. Then I grew up, and here I am 28 years old and working on my bachelor's degree, determined not to live that predetermined life. If the HH's are always being told that they won't do well in school and won't be successful (therefore driving down there self-esteem and self-worth) then they have nothing to work towards. It is already accepted that this will be their path, so why veer off course? On the other hand, the Brothers are being given that extra push by their parents to achieve and reach higher, and they too choose to stay on that path.
MacLeod's argues that the HH "internalize objective probabilities and rescue self-esteem", while the Brothers face a greater threat to their self-esteem. To me this means that the HH's have already accepted their fate and if they do decide to reach higher they won't be disappointed if they crash, while the Brothers are under the pretense that they will succeed no matter what and haven't accepted the reality that they may crash, or they they may not find success right away. When or if this happens to the Brothers it could be detrimental to their self-esteem because they were totally unprepared for this. The HH's have already protected their self-esteem by this acceptance.