If it really was that simple, it might be easier to solve this problem. But it is not that simple of course, and so life becomes more interesting. The fact is that not all boys tell lies all the time, and not all girls tell the truth all the time.
Few would deny, however, that the balance is not level. Boys tell lies perhaps one hundred thousand times more than girls do. Why is this?
There is a saying that goes: There are a billion lies, but only one truth. What this means is that to tell the truth, all you need to do is remember what is real. But once you tell a lie, you have to keep on telling more of them, to keep your story straight. And it's easy to forget the lie you told yesterday, so you sometimes get caught out, and often the way out of this is to construct ever more complicated webs of lies. So lying is very hard work.
But you could say that sometimes telling the truth is painful, so telling the truth is hard work too. It's at times like this that everyone is tempted to tell a so-called "white lie", a lie that is supposed to be for a good, unselfish reason. But we are talking here about the lies boys tell for selfish reasons, not the white lies.
If it is such hard work, why do they do it? There are two possible reasons: physical and environmental. This is because, if we are comparing the truth habits of girls and boys, the cause must logically be in the gender difference. The physical reasons would be innate (what you are born with) and the environmental reasons are learned. So the two main areas of difference are: sexual and social.
Everyone knows about the different approach of boys and girls to dating. The boy is interested in sexual conquest, the girl in relationship building (yes this is over-simplified but true in general.) The conquest mode of thinking involves ideas such as deception, trapping of prey etc etc which associate with hiding of the truth. Relationships work best in a surrounding of openness and honesty. Also, boys often feel uncomfortable talking about emotions, and would rather lie their way out of those situations. It seems there is a link between libido and lying. My grandfather, who is now very old and so is not interested in sex at all, never tells a lie. He does say, though, that in his younger days, he used to lie all the time. Could there be a connection?
The other cause of boys lying is the example of other boys, who in turn are learning from their fathers. There is pressure from these peer groups to tell lies, and many boys are told that to succeed in business, they will have to tell lies to their boss, their workmates, and their customers. Many top sportsmen are shown on television, quite openly trying to deceive referees and umpires, when it's obvious to a million viewers that they are lying. These social examples give many lessons to boys to practice untruthful behaviour.
What Can We Do?
Boys and men must be encouraged to tell the truth at all times. The best, though again rather over-simplified, way is to make clear that telling the truth is not going to cause trouble or emotion scenes, or will not be a threat. Girls need to be calm and assertive about saying this. But you do need to draw a line somewhere, so if they still keep lying to you, then I suppose you have to just as calmly dump them.