Reasonable investigation IF you are put on notice that something is fishy is an absolute element of ALL fraud cases. The elements of fraud are: (1) A false statement (2) Scienter (the party knows the statement is false) (3) Made with the intention of deceiving the other party (4) Reasonably relied upon by the other party (5) to the other party's damage or detriment. Item (4) requires that reliance be reasonable. Whether that's a new car salesman engaged in "puffing" (not actionable), a street seller of doubtful watches, or a woman who was pregnant when you married her telling you the baby is yours, the principle is exactly the same. Whether your reliance is reasonable is a question of fact and that varies with the facts of the case. But ALL claims for fraud have to meet the reasonable reliance standard.
In what other fraud case is it a requirement that the plaintiff show not only that he was deceived but that he investigated what clues there were fully enough to ensure that he wasn't being deceived?
You're quite right that the following is a condition of fraud:
[the statement was] (4) Reasonably relied upon by the other party.
However, even if reliance must be "reasonable", what I was asking about was something stronger, which is whether one "must investigate fully enough ensure that he isn't being deceived." It's not a requirement of reasonableness in any but strange circumstances that a plaintiff have done anything so extensive as a thorough investigation.
The latter is unprecedented in non-child support cases, but it is what your description of Georgia law suggested was the law in child-support cases.
So I don't think I'm switching questions. However, you're right that it's better to talk about what the law says, which is "reasonable reliance", so let's do that.
The question is whether it's reasonable for a man to take the declarations of his love partner at face value, even in face of some conflicting evidence. We can be specific. She was pregnant shortly (but not impossibly) after sleeping with the relevant man, he knew there were other guys "around", etc. But still, why should we think these clues are sufficient for a man to know he wasn't the real father for legal purposes, unless the presumption is precisely that women can be relied upon to lie?
Imagine that a man was cheating on his wife, and gave her an incurable venereal disease. If she were to sue him for this (after divorcing him), would it be a defense for him that she "ought to have known" she was running this risk, because he left phone numbers around, or had lipstick on his collar, or even made an appointment at a clinic? Can he say that he's now off the hook, despite making explicit denials to his wife that he was cheating, on the grounds that she ought to have figured out he was lying, given available clues? Is he off of the hook if he reminds her and the court that men often lie about their infidelity?
Let's just say I'm confident no court would give a man anything like the sort of latitude I'm suggesting above-- and rightly so! Why, then, is there such a wide latitude given for women who deceive their partners about paternity, even in the face of conflicting evidence? It's offensive, frankly.
I take it we're agreeing that these are usually cases in which a man didn't know that he wasn't the real father. (Given his legal action, we can safely assume that if he did know from the outset that he wasn't the father he wouldn't have supported the child.) Given that these cases also involve at least less than full disclosure from the woman (about other partners) and often outright lies (No, I never slept with anyone else!) the presumption ought to be against the woman. She ought to have to show that reliance on her was unreasonable, rather than the presumption being on the man or there being no presumption one way or the other.