There are two things that determine a dog's behavior, nature and nurture (in other words, genetics and experience). There's not much we can do about genetic influences except select the right breed for our lifestyle and exhort breeders to breed their dogs responsibly and with temperament in mind. Once a pup has been born, he is on a certain trajectory of life that is determined to a large extent by his genetics.
But this trajectory can be altered, for better or for worse, depending on the pup's experiences, particularly in an early "sensitive" period of development. With optimal experiences, the pup can become all that he can be. Under these circumstances, his social and behavioral progress will be augmented or constrained only by his genetic potential. A pup without innate flaws of temperament can become a super dog if properly raised, and a genetically challenged pup can be made quite livable. But the reverse is also true. Potentially good pups can be ruined by adverse experiences early in life and those with inbuilt character flaws can become living disasters.
The contribution of nature and nurture is thought to be about 50:50, with adverse early experiences probably accounting for the greatest proportion of temperamentally flawed adult dogs. Faulty raising practices are rife and unfortunately are almost the rule rather than the exception. In general, neither breeders nor new puppy owners understand when to start socializing a puppy, or indeed how to do it. Puppy mills and their pet store outlets are incapable of providing what is needed and some veterinarians add fuel to the fire by advising against social contact for the first 3 to 4 months of life. Their reasons center around vaccination status and the potential for disease. While it is true that attention must be paid to health aspects, it is also true that one half of the pups born in the United States do not see their second birthday largely because of behavioral problems that stem from improper nurtural experiences in early life. Clearly this matter must be understood and addressed if viable, socially compatible pups are to be produced.
When to Start
The answer to this question is as early as possible, even before a pup's eyes have opened. The process of acclimating a pup should begin at this time and continue through the first 12 to 14 weeks of life and beyond.
When pups are young, their minds are like sponges and ready to absorb almost anything we throw their way. This super-absorptive power can be used for the good, but can also lead to lifelong problems in attitude and behavior if the wrong kind of learning occurs during this period. The idea of socialization is to acclimate the young pup to people of different ages, sizes, genders, colors, and deportment while the window of rapid learning and acceptance is still wide open. When pups are first born they trust everyone and everything. At this time they should be exposed, under pleasant circumstances and with positive consequences, to people and animals of all sorts. The window of rapid acceptance begins to close toward the 8th to 10th week of life. If adverse experiences occur during this stage, the negative connotation is exacerbated and is likely to become indelible.
In socializing young pups, part of the mission is to prevent such negative experiences. I am not suggesting that because socialization is so vitally important we throw all caution to the wind and expose new pups in public places from the time they are born. This practice would this pose an unacceptable health risk to an unvaccinated pup and would not accomplish what is required. Veterinarians are right to recommend a degree of isolation but it should not be total. To have friends visit your house and interact with your pup, to pick him up, feed him, play with him, and talk to him soothingly, are all good experiences for him.
Also, it is helpful to have the pup interact with fully vaccinated and well cared for animals of the same or different species, as long as it can be assured that they are friendly. "Puppy parties," as advocated by Dr. Ian Dunbar, are helpful to teach your dog confidence and social acceptance of other people and their pets. In these perhaps bi-weekly "parties," people and their pets can form a circle of friendship, presenting mild passive challenges of novelty that can be escalated modestly from week to week. The process is one systematic habituation until all strangers and their pets are accepted as normal and non-threatening.
As mentioned above, socialization to other animals, including dogs, is almost as important as socialization to people. Generally, dogs raised in kennels do not get well socialized to people though some breeders try to offset this lack of exposure by arranging for occasional exposure of pups to a few of their friends. The latter efforts, though well intentioned, are usually inadequate and do not achieve full socialization.
Good socialization is only easily achieved in a home-based setting in which pups are allowed to move around and interact relatively freely. In short, more time in a kennel or crate means less socialization. Dogs in kennels may learn canine etiquette from their mother and littermates but may not be comfortable around unfamiliar dogs if they are not actively socialized. Such dogs will know how to signal their displeasure and ward off unwelcome strangers or submit to forces they perceive are beyond their control. Nevertheless, this is not an acceptable endpoint.*
Attempting to socialize dogs to people and other dogs after the sensitive period of learning (after 12 to 14 weeks of age) is a much less efficient way of proceeding. Being proactive about socialization and investing time when a pup is young is definitely the way to go. Sure it takes time and effort but the payoff is huge. The more time and energy you invest in raising pups the better they fare in the long run, both in terms of confidence and their ability to fit in with society.
Don't be fooled by unknowledgeable trainers who tell you that taking your dog to a supermarket parking lot to meet thousands of people will achieve socialization, or ones that tell you to take your dog to a Little League game to socialize him to children. Socialization should begin at home when a pup is young and should continue throughout life. It's always better to head a potential behavioral problem off at the pass than to try and fix it down the road. Early socialization is perhaps the best behavioral epitome of the old adage "prevention is better than cure."
It has recently been shown that a greater proportion of children who spend more than a certain amount of time in daycare exhibit behavioral problems. Daycare could be construed as the human equivalent of a kennel for puppies. In all probability, it could be demonstrated that the more time a puppy spends in a classical kennel situation, a pen, or crate, the greater the incidence of behavioral problems will be.