Understand: from the Old English "understanden"; literally, to stand under.
If Psychological Hedonism [as I use the term] is true, then it is a simple matter of fact that we cannot choose suffering over happiness; that is, that we cannot choose something we dislike over something that we like. On any occasion that we might appear to do so (e.g. choosing something that harms us excessively), it only shows that we were not, at the point of making the decision, feeling the same as we would had we then felt the (comparative) results of it. There may be some part of our brains which like other parts of our brains to be suffering, but we cannot completely like suffering, or else it isn't suffering - it would be our liking of it that was determining our preference for suffering, not the suffering experience itself. So, in short, any preference for (large-scale) suffering would not survive a vivid realisation of what that entailed.
We do not, I would say, truly understand happiness or suffering, except when we are happy or suffering. I cannot quite think how to argue for this point, because I find it entirely unobjectionable. The condition of suffering is understood only when we are in (i.e. standing under) that condition. Even Pearce opts for a simple assertion:
Even if one's ancestral namesakes [aka "younger self"] underwent great pain, then the state-dependence of memories means that much of pain's sheer dreadfulness is semantically, cognitively and emotionally inaccessible in the here-and-now. [H.I. 2.7]
Now this "vivid thinking", or simple "understanding", is one of the criteria in Brandt's definition of what it is to be "rational". So it follows, straight off, that a (Brandt-defined) rational person tries to maximize happiness. And I think it is clear that a rational agent tries to maximize total happiness, not the happiness of this or that party. Why? Well, for starters, the notion of personal identity is itself irrational [see Parfit] and in any case we use the exact same trick as before: he cannot choose his own suffering, except when he does not really understand what it is he is choosing between. But if he were to also understand the suffering of others, then no more could he choose that than his own! "Prudence" may well more common than "virtue" only because of our false beliefs about personal identity (we believe that "we" will be the ones to experience "our" future happiness, but that "we" won't experience that happiness of "others").
So my conclusion is, what seems quite likely, that if people are Psychological Hedonists, then all rational people will be motivated by the happiness of others equally (joy for joy) with their own happiness. And since, by Psychological Hedonism, happiness is the only motivator, all rational beings will be - in practice, effectively, and even by Bentham's definition - maximizing hedonistic utilitarians. And maybe even irrational beings will tend to avoid choosing that which they knew they would spurn, if only they were more rational.