Jantje is indeed a diminutive of Jan, but by no means is it exclusively feminine! It is also commonly used on males, which should come as no surprise, as Jan is an extremely common masculine name and -tje a neuter diminutive suffix.On paper, Jantje is more common for women, as it appears much more often as an official name on their birth certificate than it does for men. The reason behind this is socio-cultural.For men, the name is traditionally used informally. Predominantly in childhood, but sometimes it carries over into adulthood - by which time most men will only allow close friends and family to call him Jantje. They generally don't use it elsewhere (such as at work), as that would seem too childish and therefore unmanly (and unprofessional as well, especially in prestigious professions).In other words: there is a certain notion that diminutives are childish and unmanly on adult men. A side-effect of this is that when people outside their circle of friends and family *do* call them Jantje, it will often be in jest and sometimes sarcastically or even condescendingly.For women, the situation is different. The practice of merely adding a diminutive suffix to masculine names in order to feminize them, dates back to the Middle Ages. Back then, women were (legally) viewed as children, i.e. incapable of doing anything more than housework and raising children. As a result, it was acceptable for women to have a diminutive as their official name. After all, they were always going to be children - whereas boys will grow up to be men someday.Most women went by a diminutive anyway, even if their official name was a proper full name such as Catharina or Elisabeth, but still. It does say something about the socio-cultural mindset of the time... that merely a diminutive of a masculine name was already good enough for women, and that it was perfectly fine for women to go by a diminutive their entire lives, but not for men (at least not to the same extent).People no longer hold those views today, of course. But these practices around names endured for centuries and they are still around today (although definitely on the way out now). People have just forgotten how and why they came to be in the first place, that's all. They just see a (masculine) diminutive as an official name for a woman as normal, and equal to other names.Alright, so that is the background information on why Jantje seems prominent as a feminine name, but flies completely under the radar as a masculine name. This goes for all masculine diminutives that are officially used as feminine names, by the way - such as Dirkje, Guusje and Hendrikje.Lastly, a famous modern example of a man who was known as Jantje in childhood but as Jan in adulthood, is the Dutch singer Jan Smit (b. 1985):https://en.wikipedia.org/wiki/Jan_Smit_(singer) (in English; also see the Dutch version)He started his singing career as a child in 1996 and remained Jantje until 2005, at which time he was 20 and opted for the more mature (and fitting) Jan.The only other male Jantjes on Wikipedia are:• Jantje Hendrikx (1923-2009), a Dutch singer: https://nl.wikipedia.org/wiki/Jantje_Hendrikx (in Dutch) • Jantje Koopmans (1924-2013), a Dutch singer: https://nl.wikipedia.org/wiki/Jantje_Koopmans (in Dutch)I think it's telling that these are all singers. It seems that the world of entertainment is where an adult man can still get away with being named Jantje. You won't see a politician with the name, for example.TL;DR: the name Jantje is not exclusively feminine - it is actually unisex, but for men it's traditionally used informally, and usually in childhood at that. [noted -ed]
On paper, Jantje is more common for women, as it appears much more often as an official name on their birth certificate than it does for men. The reason behind this is socio-cultural.
For men, the name is traditionally used informally. Predominantly in childhood, but sometimes it carries over into adulthood - by which time most men will only allow close friends and family to call him Jantje. They generally don't use it elsewhere (such as at work), as that would seem too childish and therefore unmanly (and unprofessional as well, especially in prestigious professions).
In other words: there is a certain notion that diminutives are childish and unmanly on adult men. A side-effect of this is that when people outside their circle of friends and family *do* call them Jantje, it will often be in jest and sometimes sarcastically or even condescendingly.
For women, the situation is different. The practice of merely adding a diminutive suffix to masculine names in order to feminize them, dates back to the Middle Ages. Back then, women were (legally) viewed as children, i.e. incapable of doing anything more than housework and raising children. As a result, it was acceptable for women to have a diminutive as their official name. After all, they were always going to be children - whereas boys will grow up to be men someday.
Most women went by a diminutive anyway, even if their official name was a proper full name such as Catharina or Elisabeth, but still. It does say something about the socio-cultural mindset of the time... that merely a diminutive of a masculine name was already good enough for women, and that it was perfectly fine for women to go by a diminutive their entire lives, but not for men (at least not to the same extent).
People no longer hold those views today, of course. But these practices around names endured for centuries and they are still around today (although definitely on the way out now). People have just forgotten how and why they came to be in the first place, that's all. They just see a (masculine) diminutive as an official name for a woman as normal, and equal to other names.
Alright, so that is the background information on why Jantje seems prominent as a feminine name, but flies completely under the radar as a masculine name. This goes for all masculine diminutives that are officially used as feminine names, by the way - such as Dirkje, Guusje and Hendrikje.
Lastly, a famous modern example of a man who was known as Jantje in childhood but as Jan in adulthood, is the Dutch singer Jan Smit (b. 1985):
https://en.wikipedia.org/wiki/Jan_Smit_(singer) (in English; also see the Dutch version)
He started his singing career as a child in 1996 and remained Jantje until 2005, at which time he was 20 and opted for the more mature (and fitting) Jan.
The only other male Jantjes on Wikipedia are:
• Jantje Hendrikx (1923-2009), a Dutch singer: https://nl.wikipedia.org/wiki/Jantje_Hendrikx (in Dutch)
• Jantje Koopmans (1924-2013), a Dutch singer: https://nl.wikipedia.org/wiki/Jantje_Koopmans (in Dutch)
I think it's telling that these are all singers. It seems that the world of entertainment is where an adult man can still get away with being named Jantje. You won't see a politician with the name, for example.
TL;DR: the name Jantje is not exclusively feminine - it is actually unisex, but for men it's traditionally used informally, and usually in childhood at that. [noted -ed]