How much do the first few months effect a puppy?

I was just wondering last night,how much does puppy's experiences in the first few weeks/months of life effect them as an adult dog?

If a puppy was separated from it's litter at a young age would you expect them to show certain characteristics or behaviors as an adult?

Well our late Meg was handreared by me from 24 hours old, she actually had no issues and I have to say most likely was the best dog I have had the pleasure to own.

However I do think it can, as Tess came to me at 18 weeks and seems to have been effected by something in her early weeks, she shy's away from the unknown alot....like dark sheds, buildings, her socializing was very poor etc., although that has been addressed, I have to say I think Tess would be alot easier to train, if I knew what her life was like from 0-18 weeks. She can go from confident to a shaking wreck in seconds in certain situations...I have to work doubley hard with her

There are lots of studies on the effects of social deprivation on a puppy, some of them are very flawed, some less so.

Some behaviours are not just nurture related, some are the result of genetics and some a mixture.

It's referred to as the formative period for a reason.

Having said that each individual also has innate aspects of its character which may be suppressed, remain latent or not realise their potential, or be exacerbated by experiences.

However, generally, early experiences when a juvenile or adolescent are more likely to strongly shape future behaviour than later experiences once the adult dog has developed it's mature temperament.

Temperament itself then affects how much influence future experiences have on that adult. Nature/nuture.

The first few months are when the dog is changing most rapidly as that is when it is growing fastest, physically and psychologically.