How can we determine if a traumatic event in our youngest days can influence our present behaviour? Suppose that a little kid found him/herself in a traumatic situation at an age of which he or she has no conscious memories (let's say between 1 and 3 years old). For example, the kid was put in a situation where the little one experienced a lot of fear (of course you can ask if a very young kid can experience fear, but I can assure you it can). Maybe its mother wasn's paying attention to him/her while it was playing on the beach and a "mad dog" (who maybe was treated badly by his/her boss while being a puppy) attacked the poor one, while at the same time a police car with its siren on maximum volume was driving along the boulevard near the beach. Our kid grows older and when old enough he/she notices that every time it hears the loud sound of a police car's siren a feeling of "unusual" (I mean, everybody gets a little frightened when hearing the siren's sound, though less in the city than someone who never heard the sound before high fear comes bubbling up (I found myself in a very stressful situation when I was a kid of which I can remember nothing; I know this because my mother told me). The question is somewhat similar to this one but certainly not a duplicate.

Now, is there a way to determine if the story I told can be true and, if so, can we find out what the traumatic event was (assuming that no one told you, as my mother did; this is, by the way, the reason I know almost certainly that the story can be true)?
It is an interesting question, there is a lack of data to answer with certainty, but some comments can be made about the development of fear, phobias and vital events:

The consequences or effects in the present of such events would be the result of learning, conditioning, those processes would depend on:


The degree of importance that the event had, that is, characteristics such as the intensity of the stimulus and interpretation (causal attributions, control, possibility of restitution, etc.), if it is an event that is estimated may affect the survival of the individual, it will be considered as a vital event of the utmost importance. If the event produces a state of alarm or significant anxiety or if the event or related events or effects last for a considerable time.
Conditional situations or situations similar to the event, in psychology are commonly labeled as generability. In general, it is expected that there will be a conditioning to all of the characteristics of the situation. For psychologists it is usually more convenient to study adverse events and not just cases of phobias or fear. For example dog phobias can focus on a specimen of certain characteristics, however, a certain degree of emotional reaction or anxiety or fear to all dogs is expected. The event (main) and its characteristics must be studied, the scope of the generability and the intensity of the stimulus to the different specimens or similar members or similar situations.
A person can develop a specific phobia without having any direct experience. The person could learn the phobia by vicarian learning (for example, a person could develop a phobia based on observing effects on other people). In addition to dealing with fear or adverse emotional reactions it is maintained that people can learn by learning these reactions by social learning.


How can we determine if a traumatic event in our younger days can influence our current behavior?
If you do not remember the event, the effects on your current behavior would come from the process of vicarious and social learning, that is, in your current behavior you would be conditioned by the information you have received about those events with the attribution (extra) that they are about yourself. In the supposed case of your mother (or other relatives who experienced the event and remembered) the expected effect would be much greater.