No, it doesn't start with one punch. In some cases, a lot of the job will have been done by an abusive parent and started from early childhood. But even ignoring that, and thinking only of the one specific relationship with the one specific abuser, it starts very small. Hardly noticeable. A lot of the time, the very first thing is the abuser making out that the victim has hurt or upset him in some way (btw I'm using "him" for abuser and "her" for victim for convenience but realise it is often the other way around). Like "you completely abandoned me at that party, I felt so out of place" or "I hate it when your friend X comes round, I feel like I can't stay in the room, and she hates me" which has the effect of isolating the victim - they start to socially cut people off to avoid 'hurting' the loved one. They isolate themselves. Another starting factor is slight over-reactions. Let's say she spills ketchup on his shirt. A normal reaction is maybe to exclaim, try to clean it off, and then forget it and see if it washes out. An abuser, on the other hand, might throw in a couple of "clumsy cow"s, some reminders of previous episodes of clumsiness, stress how much they liked that shirt, and bring it up in company to belittle the victim... it's nothing to write home about but we're talking about an accumulative effect here. Eventually it gets to the point where every time something, no matter how small, happens that can possibly be blamed on or used against the victim, a rant starts. Swearing and shouting and put-downs and threats of leaving, gradually escalating, but nothing physical. The victim might mention it to a friend, but at this stage, friends will probably call it a lover's tiff, and offer advice on dealing with marital disharmony. Then, and only then, comes the first slap. And a lot of women have managed to retain the self-esteem and sense to walk out at that point. The ones that don't, though... they won't want to admit to it. They don't want to think of themselves as a "victim of domestic violence" and they don't want to think of the abuser as a "perpetrator of domestic violence", because those terms bring to mind the media definitions of couples half-killing each other, it's surely not applicable for one slap? They're ashamed of it having happened, they're embarassed that their relationship isn't working as well as they like to think it is. Then comes the "it'll never happen again", a phrase the victim really wants and hopes to believe - even if it is accompanied by "and don't tell anyone, because it'll make us look bad/we don't want people to get the wrong idea". But it does happen again, and again. Gradually it gets worse. "It'll never happen again" is replaced by "I wouldn't have done it if you hadn't blah blah blah and made me so angry, can't you try not to keep fucking things up?" and the reasons to not tell anyone include "I'll kill you, and your sister, and your mother, and I'll tell everyone what an XYZ you are, and you'll be out on the streets, and this, and that..." By the time any injuries are inflicted that leave a mark - let alone that require an ambulance and medical treatment and would have a hope of standing up as an assault charge - the pattern is well and truly set and it takes a hell of a lot to break it.