Without SIA, we could assume, a priori, that there would be roughly equal amounts of people in 2013 and 2014. SIA increases the number of people in 2013. So we now expect there to be more people in 2013 than 2014, or a diminishing of population from 2013 to 2014: a doomsday effect. This is especially pronounced if we assume there is a fixed numbers of observers, so moving more to 2013 actively reduces those in 2014.
Causality removes this effect, since an increase in 2013 also causes an expected increase in 2014. But in cases without causality, it is present. If we are short term simulations in 2013, we should expect that there are less short-term simulations in 2014, hence that we are slightly less likely to have simulations that continue our lives into the next year.
Without SIA, we could assume, a priori, that there would be roughly equal amounts of people in 2013 and 2014. SIA increases the number of people in 2013. So we now expect there to be more people in 2013 than 2014, or a diminishing of population from 2013 to 2014: a doomsday effect. This is especially pronounced if we assume there is a fixed numbers of observers, so moving more to 2013 actively reduces those in 2014.
Causality removes this effect, since an increase in 2013 also causes an expected increase in 2014. But in cases without causality, it is present. If we are short term simulations in 2013, we should expect that there are less short-term simulations in 2014, hence that we are slightly less likely to have simulations that continue our lives into the next year.