And it is normal.  Physiologic signs of sexual arousal occur in all children from very early infancy onward.  This arousal can happen randomly, in response to physical stimulation, or during the dream phase of sleep.  There is evidence that even very young infants find the physical sensation of sexual arousal emotionally pleasurable, and most children will learn how to create this feeling in themselves by the preschool years.  Indeed, by the age of 5 or 6 repeated, systematic, intentional masturbation is almost universal!  There is some debate over HOW children learn to masturbate.  Some think it is initially an accidental or random phenomenon - as the young child explores their own body they by chance discover that touching certain areas feels better than touching others, so they are motivated to touch those places again.  Others feel this discovery is more likely related to some discomfort in the genital area which draws the child's attention - such as a diaper rash.   It really doesn't matter how it gets started however - once begun, the behavior perpetuates itself.

There is nothing inherently harmful to the child about masturbation itself.  It neither causes nor is a sign of any physical or emotional problems whatsoever.  The best evidence for this comes from cross-cultural studies which have looked at masturbation and other aspects of sexual development in societies where both adult sexual activity and masturbation are more accepted and "public" than in our own.  In these places children are routinely witness to normal adult sexual behaviors, and they in turn they do indeed masturbate more, earlier, and incorporate sexual themes into much of their play among each other as well.   It turns out that, if anything, these cultures have lower rates of sexual dysfunction, of deviant sexual behaviors, and family disruption than we do here in western cultures. There are, however, three issues related to masturbation that parents should be aware of and think about:  The first is how their own reaction to the discovery of a child's masturbation may affect the child.  The second is how masturbation fits into their larger job of teaching children socially appropriate behavior.  The third is the need for sensitivity towards what the meaning of the masturbation is to the child, and to what that in turn might signify regarding the normalcy of the child's other sexual experiences and ideas.

It is important for a parent not to overreact to masturbation (or, for that matter other normal early sexual experimentation's such as when two preschoolers decide to "play doctor").  If a child is "caught in the act", it is best to provide reassurance, and avoid saying anything which might provoke guilt or fear.  The "repression" of masturbation - either overtly through disciplining and punishments, or more subtly through parental disapproval - can lead to harmful effects on the child's self esteem, body image, and later development of sexual identity.

On the other hand, it is appropriate and necessary for the child's sake to teach them not to masturbate in public for the simple reason that we live in a society where this is not accepted.  Teaching this can be part of a larger set of lessons regarding one's body being a private thing - there are certain things we don't do except in private, there are things others should not do to you, there are things which no one should be made to do against their wishes, etc. Thus, when you "catch" your child masturbating in an inappropriate place it is probably best to gently and calmly ask them to do that in a more private area.  If this becomes a repeated problem it may require further discipline BUT remember to keep that discipline closely focused on where he or she was masturbating rather than the activity itself.

The final concern you should have as a parent is to ensure yourself that your child's behavior in this direction does not have any abnormal meaning for them.  While masturbation itself is not harmful per se, it can (fortunately rarely) be a sign that something else is amiss.  There are essentially just two "something else's" to be on the lookout for: masturbation can sometimes mean your child is under too much stress of one sort or another, and it can sometimes be a clue to sexual abuse.  You should therefore have some awareness of normal patterns.  While many children masturbate frequently (several times per day), it is most often when they are bored and not otherwise occupied. They can usually be easily distracted from the activity - it does not have a "compulsive" quality to it.  When masturbation seems to become a compulsion it is most often being used by the child as a "stress reducer". This comes as no surprise - many adults use the activity for the same purpose.  It is not unlike other "nervous habits" in this regard (nail biting, knuckle cracking, etc.).  If your "sense" about your child is that masturbation has taken on an extreme or compulsive quality for them, once again your focus should not be on the masturbation itself but rather the stress that underlies it.  You should be trying to determine what the source of that stress is and be trying to take action to reduce it.  How's school going? Are they having trouble with peers?  Are they angry or depressed about something?  Sometimes a talk with their teacher can help uncover the problem, which will often be simple to "fix" once you are aware of it. Other times more in-depth psychological assessment and counseling will be in order.  Always, the focus should be on the root cause of the stress, not the symptom of compulsive masturbation, if you want to help your child.

Another aspect of normal patterns is that it is usually not until mid-to-late elementary school years or later that children "discover" orgasm.  Once discovered, of course, there will be strong motivation to repeat this experience and it will become forevermore the "goal" of the activity.  While this sometimes happens "by chance" in the younger child, most preschooler's masturbation is not so "directed" towards this endpoint.  When it becomes apparent that it is, it is prudent to at least consider the possibility that somebody may have taught them this - either an older child or an adult.  Luckily, the child who has been sexually abused will usually have other signs as well, the presence or absence of which can help you decide if this was an "innocent discovery" or not.  If the abuse was experienced by the child as frightening, painful, or traumatic there will be behavior and personality changes since the event.  They may have regressed developmentally or become more aggressive, anxious, or withdrawn.  They may be having new difficulties with sleep or toileting.   If the abuse was NOT threatening to the child they may not show these signs but will show an abnormal awareness or understanding of the sexual connotation of the act.   What I mean is that for most young children in our culture, even those few who "discover" orgasm on their own, they are still unaware of the connection between this activity and "adult" sexual relations between two people.  Indeed, they probably don't even realize that any one but themselves is capable of having this feeling!  It is therefore somewhat worrisome when a young child has clearly made this connection.  Such children will often signal the fact by making inappropriate and usually awkward sexual comments or jokes, by making "advances" sexually on other children (often younger than themselves) which clearly go beyond the normal "curiosity" shown by many children in sophistication, or by showing an abnormal preoccupation/interest in all things sexual (ranging from what's on TV to their own parent's sexual behavior).  If you have any suspicion whatsoever that your child may have been the victim of abuse, do NOT try to "handle it" yourself.  Seek out the assistance of a professional such as your pediatrician or a child psychologist.

In summary, masturbation is a normal and harmless behavior in childhood even at a very young age. It is important that as a parent you not react too strongly to it, and that you avoid disciplining, repressing, or making your child feel guilty about it.  On the other hand, you do have a responsibility to teach your child to limit this behavior to an appropriate time and place.   Occasionally (but not often) masturbation can be a sign or symptom that a child is under undue stress or has been sexually abused. Should such a situation ever arise, your child needs you to focus on and address the root problem, not the symptom of masturbation, in order to really help them.