Shared karmic situations fall into two subcategories: national and individual karma. An example of national karma is that you may be born in a particular country where you always have to relate with 7-Elevens, take-out pizza, and badly made cars. You end up in certain environments or worlds, but you cannot totally blame that on yourself. The whole country is made up that way.
The second subcategory is individual karma within national karma. For example, if the sewage system in your neighborhood is not good, that karma is particularly and personally yours, in a sense, because the pipes keep breaking and costing you a lot of money and effort. Another example is winding up with a bad teacher who gets grumpy because he is poorly paid by the school system. On one hand, that situation is not your fault; but on the other hand, you did end up in that particular school. You have a television network, but you have your own personal TV with which to tune in, and you also choose your own particular station. It's very simple. Environmental and individual karma complement each other; they feed each other.
From Chapter Five, "Perpetually Re-creating Suffering," in The Truth of Suffering and the Path of Liberation, pages 56 to 57.